You must first choose the custom flag you are wanting to change/undo. For example, if you have game flaged with CustomFlag6, you have to make sure the FLAG: CustomFlag6 is visible, before you can undo it. Once you select the Flag you want to change, you can unmark that flag and then choose another one to mark it with.
Hope that makes sense.
More SCID question. I am working with a database around 80 games. Trying to use Custom FLags to manage the games. So far I have been unsucessfull at changing a flag once it has been set. I have one flag to mark a game pending review (#1)and another custom flag (#6) to mark a game review completed. Any words of wisdom?