In the Mysa App, select the card of the thermostat you'd like to delete. In the top right corner tap the gear icon, again in the top right corner tap the Trashcan icon. You'll be asked to confirm this action, tap 'Confirm' to finish deleting it.
Note: If you are a HomeKit user, ensure that the thermostat has been deleted from the Home App as well. Sometimes the thermostat doesn't delete from the Home App. If you delete a thermostat, you will have to manually connect it again in the future if you decide to.