If you would like to transfer a domain currently registered with Vercel to another registrar, you can do so from the Domains overview on the Dashboard by clicking on the triple-dot menu button for the relevant domain; a menu-item button to transfer the domain out will be presented if the domain is registered with Vercel.
Due to ICANN rules, a domain must be registered with a registrar for 60 days before it can be transferred to another.
You can verify that your domain has been registered with Vercel for at least 60 days by visiting the Domains Dashboard. If the registar is Vercel and the age greater than 60 days, it is eligble to transfer.
If you run into any issues completing this process, then please contact Vercel Support using the support form available from the Vercel dashboard.