You can't set/change/delete the master password without uninstalling and installing again.

To change the master password, you need to create a backup within TB.
Uninstall TB
Start Regedit
Remove move fr om the registry the tree: HKEY_CURRENT_USER\Software\RIT
Close Regedit
Delete TB's mail directory (or move it to a different location)
Install TB
Run TB
TB's first questions are wh ere you want to store the mail directory and whether you want to use an encrypted message base
An encrypted message base means that you're using a master password and this the only moment you can set it.
The next question is whether you want to create a new account or whether you want to restore a backup. hoose the latter.

A master password means an encrypted message base. An encrypted message base means that there arent any third party tools that can read your message base. An encrypted message base means that TB can only access the message base when you enter the correct master password.
Forgetting the master password means that your mail is lost forever. (Unless you'\ve created a non-encrypted backup)