You don't.
The base url of Cloud systems is immutable and cannot be changed (yet - there is a request open for Atlassian to provide this ability and they do want to do it)
Nic is right, we are working on this functionality. Please see More Easily Change Atlassian Cloud URL Domain. There is an update from April 2017 that is promising. If you vote or comment on the bug (to communicate with the dev team) you will be notified when the ticket is updated.
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