How to do that depends on your hosting platform. Most hosting contorl panels have the ability to add “parked” or alias domains, so that’s what you should be looking for. If you have any issues with that you should contact your hosting company as they’ll b eable ot help you through that side of things.
After that you need to determine with domain you want to site served from. Which ever one it is needs to be the domain set up in your sites admin area, but I’d suggest using something like this tool to change all of the settings in your database as it’s about the easiest way I’ve found of making large-scale changes like this.
