Linux Mint, stuck in a guest account

I’ve installed Linux Mint 18.2 on an old laptop. It automatically logs me in as a guest.
This means that I have no authority to do anything at all. I’ve tried installing another OS in order to re-install LM 18.2, but my guest status means I can’t do any of that. Is there a terminal command I could use to start again please?
Grateful for any help and advice you can offer.

Have you got a LiveUSB you can boot from ?

If so, boot to it, then navigate to the

/etc/mdm/mdm.conf

file and double click it to open it. Remember you need to be navigating to the mdm.conf file on the hard drive, NOT the one on the LiveUSB.

Then post the contents of that file here.