To change a user's password in VPOP3, you should log into the VPOP3 settings, go to the 'Users' page, then double-click on the user whose password you want to change.
This will bring up a window giving you access to the user's settings. Go to the 'Passwords' tab where you can set the passwords.
Enter the new password in the Main Password or Web Password box, as appropriate, and confirm the same password in the 'Confirm…' box. Then press the Submit button.
Users may have one or two passwords. There is a 'main password', which is used for POP3, IMAP4, SMTP etc and optionally a different 'web password' which is only used for accessing Webmail. This allows administrators to configure a password in the user's email client software which they do not need to remember, for POP3, IMAP4, SMTP, but have a more memorable password for accessing Webmail which the user can change if they wish.