MyBB Community Forums

I have bought a domain name cryptoforumtalk.com

I would like to change sub domain https://cryptoforumtalk.createmybb4.com/

Appreciated your help.

your forum's admin control panel should be having a link for domain manager.
you have to use that domain manager. post at createMyBB for further support

Have you updated your name servers if needed?

Admin panel > Configuration > Site Details