Modify the Domain Owner
Go to Domains > Details of [example.org] - 🔎 > CHANGE (opposite Owner). This brings you to a form where you can fill-in the data on the new owner.

A confirmation e-mail is sent to both parties. Check the e-mail addresses before launching the operation.
Note
It is not possible to initiate a domain ownership change operation for a domain that is close to expiration. Renew the domain before initiating the ownership change.
There is a charge for this operation for some extensions, including: .am, .be, .me.uk.
Tip
If you just want to update the address, phone number or email address, you can simply update owner information.