Editing posts has a time limit, about five minutes, if I recall correctly.
Editing tags should always be possible. Look at your tags for each topic and see if there isn't a red X. If so you should be able to click it to delete the tag. You can then replace it with what you want.
I haven't done this in ages, but I think that's how it works. You cannot delete or retag someone else's topic, of course.