16.04 - Login to Ubuntu after update if you don't know user name and password

I bought Laptop Dell Inspiron 15-3552 4113/SDPP/2015 5100 12439497278 Service tag (s/n) 5PQ5LJ2 MFG YR 2017 with Ubuntu 16.04.6 LTS already installed and after one update on screen apair login user name and password which I don't know it. I check with shift/esc/space to reach at Grub menu to change the user name and password but nothing works. Please help me!