Thanks for the reply. I've connected my TomTom to my computer, opened the map folder, but can't find the sort of file you refer to; that's the problem. Am I missing something? Doing something wrong?Use Explorer. Go to the specific map folder. Look for a file called .wal-mart.ov2 (notice the . at the beginning? The name wal-mart may be slightly different).
If you see such a file, delete it.
And thanks also to mikealder. I'm trying to do this on a Mac.