Ah, no, sorry, I'm afraid Damien has the right answer for server versions of JIRA (although you also need to change your network so that it resolves the new url correctly), but not Cloud. You mentioned Cloud in the tags and you've got an atlassian.net address, so I'm pretty sure you're on Cloud.
You can't change the url there easily, you have to get a new one and migrate over to it. See https://confluence.atlassian.com/cloudkb/changing-the-cloud-instance-url-address-or-migrating-to-a-new-cloud-instance-691011835.html
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