Hi @avinash ,
I don't think it's possible to change the "id" of your subdomain, since we are talking about a subdomain (different "folder" within the atlassian.net domain). You could start a new one from scratch and move everything from the old one to the new one, or change the "organisation name". For the latter:
As you can see from the image below, "Your organisation name is displayed in headings and emails sent to managed accounts. It will usually be the name of your company or organisation".
Hope that helps!
for changing the specific sub-domain like old.atlassian.net to new.atlassian.net there are reports in Community that Atlassian does this as a one-off upon Support Request - but only on paid plans (Standard and above).
In case you are on a Free plan there are options described in Knowledge base:
For all of the other options like renaming an Organization please refer to what Alex already suggested.
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