After searching, I have found no way to change your password from anywhere within the game.
There are no external tools to change the password other than hexediting the actual save files.
For the moment, I'm going to say it's not worth your time to try and change it, especially if it's a valuable password. Just delete and start again.
By the way, Uplink doesn't delete your save files when you retire an agent, it merely moves them to a "deleted" directory, so if you're very concerned, you should delete them manually.
- On Linux your save files are located in
NAME.tmp (replace NAME with your profile name, obviously).
- On Windows they should be located in
/Program Files/Uplink/User/ or something similar.
- I don't know where they might be on Mac.