Password Problem

I look after the computer network at the local literacy council. About 3 years ago a retiring lawyer presented us with 4 computers 3 running windows and one Ubuntu 14.04, they were all sanitized and all in good condition, we gave them to clients who could not afford to buy a PC. A few weeks ago the one running Ubuntu came back. Someone had played with i (that's a kind description) it took me an afternoon to get it working again and I'm using it now. Unfortunately the client passed it another person and no one admits to the damage. The lawyer, one of the good guys) has since died. We do not have a password for it, I can't update it or sign in without a password. Is there a way to change the password without the old one.
I realize this is an awkward request but I would like to update and carry on using the PC
Regards
John Morris
sirrom