To make a register user an admin, use the Web terminal and run the following command:

sudo -E -u www-data php artisan user:admin username_here
Found username: girish
 Add admin privileges to this user? (yes/no) [no]:
 > yes
Successfully changed permissions!

Further administration commands like removing a user, removing unused media can be found in Pixelfed docs.

The admin page is located at https://pixelfed.example.com/i/admin/dashboard.