Hi,
An issue marked FP will stay marked FP (and so not included for instance in your Open issue counts) as long as the issue tracking mechanism finds that they’re still present in the code. For details, see the ‘Understanding which issues are “New”’ topic in the docs.
As for how to mark them FP, if you have the Administer Issues permission on the project, that option should be available to you in the interface:
Ann