To delete the backups, go to iTunes>Preferences>Devices. All iPhone backups will be listed here. Highlight them, then hit Delete. 

But if you're using a Mac, I would first try to see if you can find the password in your Keychain (from [discussions.apple.com][1]):

>- Go to the "keychain access " app.
>- Under "Keychains" select "Login"
>- Under "Category" select "Passwords"
>- Scroll down and look for "iPhone Backup"
>- Double click and a window opens
>- Check the box that says "Show Password"
>- A window pops up asking for your OS X account password
>- Type in that password and voila!

  [1]: https://discussions.apple.com/message/21385292#21385292