How to replace sink shut off valve
This doesn't appear to be PEX pipe but a 1/2' or 3/8&quot; compression inlet. Remove the valve by unscrewing the red part  and take it to your plumbing supply store so they can attempt to  match the compression face and threads. If the compression sleeve and nut come off the pipe easily, take them with you too.
Your valve stem is PVC so I'm curious as to why you want to replace the valve.