8 inch screen resolution

What is the 8 inch screen resolution mine says 640 × 480 , looking at the forum people are saying 800 x 600 , new to using reaspberry pi so how do I change it to 800 x 600 as no option on screen settings

What specific 8 " screen?
One way to set screen resolution in Raspbian is with a config.txt file edit. sudo nano config.txt, it should be on the boot partition if memory serves me. Have a look see here, https://www.raspberrypi.org/documentation/configuration/config-txt/video.md