To transfer a domain from one registrar to another, you need to generate an authorization code from the current registrar, and then send that registration code to the new registrar. Using this unique code, the old registrar will allow the new registrar to take over the domain registration.
You can get an authorization code to transfer your domain (also sometimes called EPP) from the domain manager, under Store → Domains in your dashboard.
The transfer usually takes a few days, and when it is complete, your domain will no longer be registered with WordPress.com. Keep in mind that this doesn’t affect your WordPress.com blog. Your blog will still be available at the default address.