I have to replace a 36" x 36" 3M decal due to a scratch. What is the recommended procedure to remove the existing damaged decal?

In most cases it is very easy to remove adhesive films. Peel away the film from one corner and pull the film from the glass. If any adhesive remains on the glass it can easily be removed with a spray of soapy water and a single edge razor blade. Blades cannot be used on plexiglas or polycarbonate sheets to remove film.