First, request the transfer of your domain name at Settings > Domain names > New domain name.
After your payment, you can enter your transfer code (also called AUTH code, EPP code or token) at Settings > Domain names.
Please double check that your transfer code is correct before you confirm. Transfer codes can have upper and lowercase letters, numbers and special characters. Most errors occur when typing the transfer code manually. Make it easy for yourself and use copy and paste!
If you don't have your transfer code, you can request it at your current registrar. This is the company where your domain name is registered at the moment.