To change your subdomain, head to your Settings page.
Scroll to "Danger Zone" and click on "Change subdomain" and enter the name of your new subdomain.
Note: Changing your subdomain will break all of your links. The easiest time to change your subdomain is before you've published anything.
How do I change my subdomain without breaking links?
To change your subdomain and redirect all links to your new URL, click on "Change subdomain without breaking links".