You can actually reach out to Atlassian support and they can change it for you. Note there is a limitation on them doing this. I have heard once and done kind of thing.
Hi @Bhoomika ,
On Cloud you don't change the base url. You register an instance with a name and that's the name.
If you are on Server however (you mention Cloud in your tags) you can do it with these steps:
https://confluence.atlassian.com/adminjiraserver/configuring-the-base-url-938847830.html
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