How far off from dock height is it? Look under the truck where the box joins the frame, look for a 2x4 or 4x4 between box rails & frame rails. Probably not there. Seek out dock high straight and compare theirs to yours.
Loosen the box bolts (big U bolts) jack up box insert appropriate boards lower and tighten.
Based upon unmentioned skill set it might be easier to pay a shop to do it.
The key is to compare a dock high straight to yours. So you can see what's needed.
To much is undefined in the post to do more than guess.