You have to change the records at the nameserver that handles your domain. This may be at the registrar, or (tricky) your server.
Unfortunately this sort of task doesn’t have a ready made tutorial, it’s something you work out from knowledge of how DNS works, how to change DNS records, how to migrate services, etc.
To be blunt, if you don’t feel up to it, hire an expert sysadmin to do it. If that is not possible, then at least practice with a dummy server before you do the migration for real.