8860 Change Web Server name - Linux

Hi, I have a dedicated server (linux/apache) and the main server name's domain has expired. This is messing up my email - other servers are rejecting my email because the servers domain has expired. I just want to change the server name and mail server name on my dedicated server to one of my domains that has not expired. I am sure this must be fairly simple, but I dont want to mess anything up and am more than happy to pay someone who knows what they are doing. Thanks so much.