Phone owner change

[Roma1198]

I just purchased an used xperia z5 compact. After restore to factory defaults i was not able to start the OS. Luckly i contacted previous owner and logged in on his account and add me as a user. Now he has changes his password and i am looking a way to make myself owner of the phone. Is there a way to do that? I did not find any info on that. How can i make myself owner and dwlete previous one. Thanks in advance for help.

[Uliwooly]

@Roma1198

That's quite odd, how did he added you as a user? 

[Roma1198]

He gave me his credentials and i signed in and added myself as a user.

[Roma1198]

After that he changed his google pass

[Lord_viridis]

What a wierd way of getting around this.
What you(they) should have done, was log in using his credentials, gone to security settings and disabled my Xperia protection for reset devices, and disabled the Google boot security.
Once done, simply repair installation using Xperia companion and set it up in your name, not his

[Lord_viridis]

But anyway, if you've logged in, and got past my Xperia,
Go to settings, accounts, Google and make sure your account is the only one showing.
Also check other accounts for any remnants of his info.
Then go security.
Turn off my Xperia
Turn my Xperia back on and confirm YOUR email is the one displayed.
If so, it's in your name.