Well, that depends. What kind of tonneau cover is it ? Some are secured to the front, side and rear truck bed edges with just velcro and the cover can be pulled off of the velcro strips. If youre then looking to remove the velcro you can GENTLY peel the velcro off and remove any left-over adhesive with any over-the-counter adhesive remover. Some other types of covers install with metal snaps ( like a button ), the cover just unsnaps from each button, unfortunately though, the part of each snap remaining on the truck is attached by a sheetmetal screw ( body work and paint are required to repair this damage after removal. Finally, the best ( and most expensive ) type of tonneau cover is set up on a "rail" system which is clamped to the inside of each truck bed rail. The cover has buttons that slide into grooves on each rail and, usually, velcro securing it at each end. If you have this type, peel the velcro loose from the tailgate end, lower your tailgate and slide the cover out of the rails.
Hope this helps, let me know if you have any other questions. -- Joe