[2023] Xbox to pc

You cannot merge two accounts.
You can read on "unlinking" Xbox account on our official forum here: https://forum.warthunder.com/index.php?/topic/543390-unlinking-xbox-account/

Originally posted by Gonzu: So there is no way I can move the premium vehicles from xbox to steam, and keep my current ones?

There is no option to do this, sorry.
You can wait for another "wave" of possibility to unlock xbox account and make it PC, but that's all. There was no, and AFAIK there will be no option to merge two accounts.