The lexicon file used by the spell checker is never changed. Any errors that
have been introduced are in your custom dictionary (custom.dic by default)
and can be edited out. You should find a button on the Spelling and Grammar
tab of Tools | Options that gives you access to this dictionary; the
interface varies somewhat among versions, but you should be able to figure
out how to do it. If you find that the offending word is not in the custom
dictionary, it may be an "incorrect" word that is allowed by the lexicon
because it is in fact acceptable in some contexts. If you'd like to have it
marked as incorrect anyway, you can add it to an "exclude" dictionary; see
http://word.mvps.org/FAQs/General/ExcludeWordFromDic.htm