Edited for extra detail.
Here's a quick, easy and efficient 1 can (or 3 bottle) AHC fluid exchange/bleed procedure. By following this process you are very unlikely to get yourself into trouble by accidentally bleeding too much fluid. Furthermore, it's not at all wasteful of new fluid as you remove a considerable amount of the old fluid/gas/air and recharge the whole system with fresh fluid in just one cycle. No wasteful start/stop/bleed/start/stop...... process.
The benefits of periodically changing out the AHC/AVS systems' fluid can't be overstated as this mineral based working oil degrades with time, temperature and use. Fluid viscosity changes, anti wear, anti rust, anti foaming, anti oxidation additives deplete and the fluid picks up particulate and other contamination. Nitrogen from the accumulators permeates into the system over time and virtually all hydraulic systems will allow air to come out of fluid solution and form bubbles with use and time. The combination of degrading fluid and entrained gases affects damping performance and ultimately can't promote the maximum service life of the system. FWIW I change out the systems fluid at half the recommended service interval, or about 30k miles/3 years.
Materials required. One can of fluid (Toyota suspension fluid, AHC 08886-01805) or three of the new 1L bottles (08886-81221), clear pvc pipe that's a snug fit (3/16 ID) on the bleeders, 10mm spanner/box wrench/crows foot and a suitable waste fluid container.
This process assumes the fluid level in the reservoir is between max and min when the vehicle is at Normal height before commencing and the 5 bleeder points aren't frozen due to corrosion. If corrosion is an issue then consider spraying a penetrating oil (PB Blaster etc) on the bleeders several times over a few days beforehand and use a good quality 6 point 10mm socket wrench. If you can't open the bleed points due to rust then an option would be to drive the AHC system down to the bump stops (lower then L and returning the maximum amount of fluid back to the reservoir for removal) in Active Test Mode and remove/replenish the fluid from the reservoir and not attempt bleeding at the 5 points. Not optimal but probably better than doing nothing. Always clean around the reservoir cap and the spout of the AHC fluid can before opening and avoid introducing any contamination into the reservoir.
This maintenance activity needs to be done with the vehicle on flatish, level ground and not on a lift or with the wheels off the ground as you need the vehicle's weight to compress the shock actuators down to the bump stops to remove the maximum amount of old fluid. Don't get under the vehicle, you access the bleed points on the outer frame rails.
1. Lower the vehicle to LOW and wait 30 seconds for the AHC system to settle and then turn the vehicle OFF. Do not restart the engine until step 8. The fluid in the reservoir will now be at its highest level. Remove the reservoir filter screen and using a turkey baster or similar suction device remove as much of this old fluid as practical from the reservoir. The amount removed should equal, or be slightly over, 1 liter [if you were between Max and Min at N].
2. Replace the filter screen and pour the entire contents of a 2.5 liter can [or 2 1/2 x 1 liter bottles] of new fluid into the emptied reservoir. It will be very close to completely full.
3. Now we know that the reservoir's "normal" contents is ~ 1 liter (vehicle in L when fluid level is correct at N) and you have just filled it with 2.5 liters so you don't want to drain/bleed more than 1.5 liters or else you may end up with insufficient fluid on completion.
4. Starting at the height accumulator (12 inch long cylinder on DS outer frame rail, between front and rear damper assemblies) connect your drain tube, crack the bleeder and commence draining fluid, closing the bleeder as fluid flow stops. You should drain ~0.3 liters.
5. Move to the front LH damper assembly, connect tube, crack the bleeder and drain until the front has dropped down to the bump stops. Both sides of the vehicle will lower together (because they are hydraulically connected) and you should drain ~0.4 liters. Again close the bleeder as fluid flow stops.
6. Move to the rear LH damper assembly, connect tube, crack the bleeder and drain until the rear is close to/on the bump stops. Both sides will lower like the front did and you should drain ~0.3 liters. Again close the bleeder as fluid flow stops.
7. Now you should have drained very close to 1 liter (0.3+0.4+0.3 for healthy damper accumulator spheres; if you have degraded or blown spheres then you'll likely drain less fluid as there's limited/no nitrogen charge to expel the spheres fluid contents) leaving you about 0.5 liters "excess" in the reservoir.
8. Start the vehicle, let it idle for 30-60 seconds and then press the UP button to raise to NORMAL height. The whole system will refill and recharge with new fluid from the reservoir and the level in the reservoir will be drawn down to slightly over the max line if everything has gone as expected.
9. Turn the vehicle OFF and do a small, quick bleed on the PS front and rear damper assemblies just to check that there isn't any gas trapped. You don't need to remove very much at all from the PS as essentially all available fluid (there will always be a small residual in each shock actuator/damper that we can't access) is forced out when you do the DS dropping the vehicle down to the bump stops.
10. You're done, confirm that the reservoir fluid level is correct - between max and min at N height with the engine running. There is nothing wrong with leaving the level slightly over the max line.
As long as you don't drain more than 1.5 liters you can't mess up and accidentally draw air into the pump/system through the reservoir or be left short of fluid.
This approach is simpler than others, but slightly more elaborate than the FSM. It is intended to remove as much old fluid as practical in a quick and efficient manner. It doesn't waste new, fresh fluid like the drain/start/stop/drain/start etc method. Hopefully this method fills an information gap by associating approximate quantities drain to the height accumulator and the front/rear hydraulic circuits and gives people the confidence to undertake this simple yet critically important maintenance procedure.
