Users
RUN sites do not have a single owner. Rather, there can be multiple Admin users. Transferring control of a site involves adding people as users who you'd like to take over management of the site, and then, removing any users you'd no longer like to have control of the site.
You can not remove yourself as a user, so another Admin will have to do this.
Payment
All Admin users have access to the payment settings where the billing contact and payment info can be updated. The billing and payment settings are controlled via the Stripe portal and are separate from the RUN site users.
Teams
You may also want to remove the site from any Team accounts controlled by the original set of users.