If you go to Menu-> Settings-> Accounts&sync and tap on your Google account, you can remove it there. Also Settings-> Applications-> Manage applications - choose the google app (mail or talk) and you can clear data there - that will remove the account.
Then add it back with the correct info by launching Setup from your application drawer, and double check it with Accounts&sync again.
Should work - but _will most likely_ cause up to an hour or so delay to be re-recognized by the Market.