Just purchased a new armrest with cupholders but I can't seem to find any guide on how to change it..

 

Rule #1 - ALWAYS check the Stickies.
This is so fundamental, there are few other rules for seaching the Forums. LOL

The X308 Workshop Manual is right there in the first Sticky.
Having just been in there to repair gear selector unit on my car (cup holders next), there is one aspect that I missed before disconnecting battery . . . drive both seats fully to the rear . . . and you will find out why if you remove the full length centre console.

However, do check . . . because, if you are only removing the cubby box and its upper armrest, this is a real walk in the park.

Best wishes,