Hello, i have a razer deathadder white edition, how do I set side buttons to press Back on the keyboard ? So whenever I watch videos or browsing, i can just hit the button instead of moving my cursor to the back arrow.

set the side button function to back in razer synapse

have you try using the Razer Synapse application?

I did, I set it to Keyboard Function > Backspace, but it doesn't work.

https://dl.razerzone.com/master-guides/Raze...-0000083-en.pdf

should be under Mouse Function

maybe try updating the app or reinstall it

also check whether got update for ur mouse

Still doesn't work after updating and restarting my pc, it is Navigation > Backspace right ?

what browser you use? alt+left arrow for previouse and alt+right for foward.

Chrome, how do I set alt+left arrow on the mouse's left buttons ?

under keyboard function.
choose navigation.
btw i don't have/use razer devices so i can't see directly the application.

It's not keyboard backspace, but mouse function "back". Pressing "backspace" on the keyboard doesn't go to previous page in a browser