On Cloud, you don't. The base url should always be the url you access Confluence on. On Cloud servers, the base url is locked to the url of the service and hence there's no need for you to change the base url, because that would just break your site. The url for your service cannot (yet) be changed (I don't know if Atlassian are planning to allow changes any time soon, but if they do, you can be certain that the base url will remain locked to it)
If you need to change the account url, then you have to set up a new Cloud service with a new url and then migrate your data to it,
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