Yes you can delete any of the backups in iTunes from iTunes/Preferences/Devices. Then you can change the password for your future encrypted backups by clicking the Change Password button. If your Old Password is filled in, you can enter a new password and verify password - make sure you save it in your Keychain. Now you can create a new encrypted backup in iTunes with your new password.
Assuming you saved your old password in Keychain, you can look it up in the Keychain app from your Mac. Open Keychain, make sure Keychains in the left pane says Logins and Category says All Items. Then, look for IOS Backup, double click on it, click on the Show password box and enter your keychain password - which could be the same as your user account login.