@eflyguy is right, you will need a 3mm hex key to remove the old pulley and install the new one.
I find it easiest to just lay a flat mirror on the tray (or you can take the tray out to give yourself some more working room) then you can see what’s happening underneath the laser arm.
There are instructions below for removing the carriage plate and belt, and you will follow them to take it off. Then use the hex key to unscrew the pulley. It slides in a little slot…you can see it underneath if you use a mirror.
Put the new pulley in, approximately where the other one was in the slot, and then re-attach the carriage plate.