Are you using Cloud or Server? For Cloud you would have to sign up for a new instance and migrate from your old one to the new one per this KB, https://confluence.atlassian.com/cloudkb/changing-the-cloud-instance-url-address-or-migrating-to-a-new-cloud-instance-691011835.html
If you're using Jira Server, it's possible you would not see this in the system admin menu if you were just a Jira Admin and not a System Admin. Managing global permissions explains that this is expected as only System Admins can edit this system setting.
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