How you remove your corner guards depends on how they are attached to the wall. There are three types of fasteners: self-stick tape, adhesive, or nails.
How to Remove Corner Guards with Self-Stick Tape and Adhesive
Use a putty knife and scrape between the tape or adhesive and the wall from the top to the bottom of the corner guard and on each side.
There will probably be tape/adhesive remaining on the wall so you can use the putty knife or fine grit sandpaper (200 grit) to remove the remain bits.
How to Remove Corner Guards with Nails
Corner Guards with nails can simply be pried out by sliding the back of a hammer close to each nail and pulling them out. In order to protect the drywall, you can put a piece of wood or shim between the hammer and the wall so that the pressure against the wall is not concentrated in one spot.
If you damage the wall it can be repaired like normal drywall repair.