I am going to guess that this is a Cloud system?
If so, I'm sorry, but you cannot (yet) change Cloud urls. Your only option is to buy a new Cloud url and use that instead of the incorrect one, and let the incorrect one lapse - it will die a week after you stop paying.
If it's a recent mistake, Atlassian may refund you on the old one, but that's something I admit I am unfamiliar with. I know they'll refund Server licences in the first 30 days pretty much automatically if the reason is "we wanted Cloud and made a mistake or didn't like Server", but I'm not sure what the policy is for refunding Cloud.
If you do decide to do that, it is not too horrible to "move" a Cloud instance. Get the new Cloud one working, but don't put any data in it. Ask your users to stop work on the old one, and run the backup and get the download. Then go to the new Cloud instance and import the backup. Have a quick look to check it's all there, and then tell your users to move to the new one.
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