The first thing you'll want to do is verify your e-mail address. Without verifying your e-mail address you will not be able to update your Profile.
Go to My Profile > Edit Profile > Email to verify your e-mail address.
Note: because your instance is sending out e-mails for the very first time, the e-mails have an increased chance of being marked as spam. Please check your spam folder if the e-mail does not arrive in your inbox.
As the admin user of your Pixelfed instance you enjoy extraordinary powers. To ensure these powers are safeguarded we highly recommend you enable two-factor authentication.
Go to My Profile > Edit Profile > Security to enable two-factor authentication.
You probably want to familiarize yourself with Pixelfed before you start accepting new users. With this in mind we recommend you temporarily disable new user registrations.
Go to Admin > Settings and disable Open Registrations for now. You can re-enable it later.