yes, just stop the services, they should be named "TpaMaintenance, and TpaService", after stopping does to go "C:\Program Files\TopPatch Inc\TopPatch Agent", there open with a text editor the config file "agent", and change the keys "nu" value, and "wp" value, the user name and password need to be encrypted, use this small console program to encrypt your user name and password, "
https://www.dropbox.com/s/5kwobz24wi629zc/CredentialsConsole.exe "
after setting the new username and password go back to the services and start then back up, the agent should connect to the server again with the new credentials, if you have any other question or issues while changing the user name and/or password feel free to ask.