For server, it's in the general configuration.
For Cloud, you can't.
In both cases, the base url must match the url upon which your users are using JIRA. You can't change the url for Cloud, so you have no need to change the base url (ok, you can, but technically, that's "copy old site to new one and delete the old site", not a simple change)
The doc you've referred to is about naming your application, not the url it runs on. By default, JIRA is called JIRA, but there's no harm in calling it "Bob" or "My issue tracker" or "Slartibartfast" if you feel like it.
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