You'll need access to the osticket database. Hopefully you have a GUI something like PHPMyAdmin, otherwise you'll need to know mysql commands etc.
Find the osticket database you're using. Navigate to the ost_staff table. Find your user in there and edit them. Change their put in the MD5 hash of whatever password you want.
You can create MD5 hashes of passwords here: https://www.md5hashgenerator.com/
Edit the row of the user you want to modify. Put the hash you generated into the "passwd" field. Make sure in the "backend" column it has nothing in it. If you were using LDAP authentication or something change it to blank.
Save & done.