I used PostgreSQL to build the database of an online questionnaire I wrote (and in a couple of other projects), so I have plenty of experience with it. In this particular case, I don't see why a simple
UPDATE <your table> SET <username> = 'admin' WHERE <row> = <whatever>
shouldn't work, so I guess I will need a bit more information about the project before I will be able to help you.
Best,
Anton