The quickest and simplest way to login as admin to your site is from the list of sites, by clicking the "Login As" button, as shown in the screenshot below:
Before you login as admin to your site, you will be prompted to enter your password, Yubikey, or PIN:
Note: The Login As feature is a security-sensitive feature and therefore always requires input of account password or PIN.
Important: If you're not logged into your site automatically after entering your password. Yubikey or PIN, please synchronise / retry communication with the site. See here on how to do that.