The non-modded version of the game is 100% free and can be found here: http://www.scpcbgame.com
As for installing the Ultimate Edition (which adds some useful SCPs and items) I don't know how to help sorry.
i'm not sure what to flair this as so i hope imagery is good enough
link: https://www.vrchat.com/home/world/wrld_969476f0-6924-4e7e-9100-b90b82baa4fa