I hit a curb and now steering wheel is not centering I checked under my excursion then compared with my neighbours F250 and the only damage is a dent on the damper arm cylinder.
How do I adjust?

 

I do not believe they are adjustable. It may have to be replaced.
I would also suspect something in the steering is either bent or broken.
Not quite sure what you mean by not centering.
If you mean the steering wheel is not in the same position when going straight ahead, something is bent or out of line.
If you mean the steering wheel does not return on it's own ,there is a problem with the centering spool in the steering box, and may require that the box be replaced.

 

I have been regular customer of the Akins Collision Center and it provides expert services that will makes your car running smooth as ever and looking as brand new. The dent on the damper arm cylinder will be easily removed by there experts. so, I suggest you to must visit there.