I had not used my password ever to log in and so when I needed it to add software. I could not seem to get it right. So I was wanting to have a simpler way of resetting a password than re-installing UbuntuMate. Sorry to have bothered you unnecessarily. I was surprised to see that the updates did not seem to need passwords also.
It sounds as if you would like to reset your password. Here are a couple of links to assist you in that task. -
and -
Good luck @grcoukell
Edit: Also see @pavlos_kairis answer, post #2 above for a simple and direct method.