On an Android based phone it happens sometimes that you unintentionally add a word to the dictionary (also called the “user dictionary”) that you didn’t mean to add. On the HTC Hero (or Eris, depends on where you bought it) there isn’t apparently a way to edit this dictionary – only delete it.
I’ll repeat the advice I found here, presumably based on an HTC support request:
Thank you for your questions about how to delete an entry in the dictionary. We are more than happy to help you with this situation. In order to clear out a misspelled word you accidentally saved you must clear out the entire dictionary. To clear the dictionary you press Menu then go to Settings > Applications > Manage Applications and find the “Touch Input” program. Tap on the Clear data button.
Shameless plug: If you’re an Android user, check out my app, Dindy.