You can't (easily) in 1.84 and below.
With this version, the only time the saved password will get cleared is when authentication fails. So, to force this, you can change your password on the host, then login again. When the login fails, the authentication box will display and you can enter the new password.
I *do* have a new beta out that allows you to clear the authentication via a menu. You can download it here:
http://www.celestialsoftware.net/telnet/AbsoluteTelnetBeta.exe
The option is under Options->Properties->Connection->SSH?->Clear Authentication