The simplest way is to create a new site:
sudo systemctl stop piaware
sudo rm /var/cache/piaware/feeder_id
sudo systemctl start piaware
This will generate a new feeder ID and a new site. Then claim the new site with the right user.
If you want to reassign an established site, retaining history, you’ll need to email adsbsupport@flightaware.com, it is a manual process.