Change Your Subdomain
On your website's Domains page, press the Edit Subdomain pencil in the Free Subdomain section, type the new name and save. The old subdomain is released the moment you save and links to it stop working. Changes are free and unlimited.
Updated
Open your website's Domains page and find the Free Subdomain section. Press the Edit Subdomain pencil button, type the new name and press Save. Your website is immediately reachable at the new address. You can change the subdomain as often as you like, on every plan, at no cost.
A subdomain is 8 to 60 characters of lowercase letters, numbers and hyphens. If the name is already taken or reserved, the dialog says so - try another.
The Old Address Stops Working Immediately
Your current subdomain is released the moment you save. There is no redirect from the old address to the new one - anyone following an old link sees that the site is gone, and the released name becomes available for others to use. Update anywhere the old address is published: social profiles, email signatures, printed material.
QR codes count too. The QR Code page for your website encodes its address, so after a subdomain change, open the QR Code page and download a fresh code - any previously printed QR code still points at the dead address.
Deactivating Instead Of Changing
The same section has a Deactivate button. Deactivating takes the website offline at its subdomain but keeps the name reserved for this website, so nobody else can take it. Press Activate to serve it again. Changes take about a minute to reach the whole network.