Admin user name change.. gone wrong

Hi guys can anyone help me out? I've just bought a MacBook Pro from a friend (running 10.13.6.)

The user name and password were set to User and password

I wanted to update the user name but when doing this the Mac hung and now neither old or new password can not be used when installing software.

When I turn on and login the old password is accepted (User / password) however when trying to installing neither new or old are accepted and I can not install anything.

Any thoughts how I can fix this?

Cheers,

dEVS!

Have you tried going into system preferences and adding a new user then signing in as that user? Might be a corrupt user profile.

Repair Disk Perms ?