If you have a removable top, invert it on a few blankets, put on tennis shoes, and do the happy dance (gently) on it.
If you have a fixed roof, it is oil-canning, and you need to bend it beyond the starting point to get it to stay. You may want to jack some wood blocks into it from inside the truck (with a floor jack) to push it back out. You may have to fab some plywood pieces to match the shape of your roof. Take your time with this.
I had a guy weld extra roof supports in my FJ55 roof because it still oil-canned after pushing out dents.
These extra supports are hidden under the headliner and should be glued to the roof metal with urethane so that they don't rattle in the future. Get some thin metal rods, bend them in an arc to match the roof, weld the ends to the rain gutter area (inside the truck) and then glue the inner areas in places.
Steve