The debugger shows everything there is to show in a running program, which is very dependent on where the current position in the program is. Debugging can also be used to alter the report behavior, without changing it's coding. When an authorization check is done, the outcome could be negative, in the debugger this outcome could be cleared, making the report continue as if the check passed. Hence debugging with change authorization is a bit of a no-no in production systems. Actual changes are logged in system logs, so there are ways to work out what happened - make sure you check this when trying to solve a production issue you can not reproduce.
Double click on the variabele of your interest, which will make it appear in the bottom or right pane. Click on the pen besides the variabele to make it's value changeable. Internal tables can also be edited, including deletion and insertion of lines. Can you not find the pen icon? Debugging and debug changes are authorized separately, it could well be that you can debug, but you cannot change values because of authorization.