^don't double post, edit your previous one...it just makes more work for mods.
You could use paint or photoshop to edit out the D- and put a b+, it would be really easy if he got a b+ already on the report in something else, then you could copy that part, and put it in a different spot. But i would advise against doing this, as it is editing an important document, it could be considered against the law, and he got the mark, that's his problem.