Apparently, the only way one can delete a typo in Safari autofill data is by deleting all data related to the password/credit card/website.
This may be OK for a password or credit card, but I have an instance of a form field where I have several hundred correct options, with a handfull of incorrect options. I do not want to restart this from scratch.
This data is stored in ~/Library/Safari/Form Values, but is an encrypted plist. It sure would be nice if one could edit each entry manually. It should not require Developer Tools and/or third party utilities to do this.