You cannot reset the password. You must remember (or guess) the backup password that you entered the first time you made an encrypted backup. It is a password that you have used before, because you entered it (even if you don't remember doing so).
If all else fails Elcomsoft makes a decent password cracker, but it is a professional tool and is priced correspondingly. They do have a free trial version that will give you the first two characters of the password which may be helpful.