To join an agency you do not need to delete your personal account. To join an existing agency follow these steps:
If you're being invited to an existing agency
If they haven't already, ask your agency to invite you to join them.
You should get an email to join their agency, accept the invite by clicking the link in the email.
Tern will walk you through joining the agency.
Optional: if the agency is paying for your subscription you can cancel your personal subscription so you're not double paying.
If you want to create a new agency
Go to your user settings (click on your avatar in the top right)
Click Set Up Agency
Enter the details about your agency
Decide if you want to purchase seats for your agency now or later
If you want to move yourself to a paid agency seat you can assign yourself a paid seat. You should then cancel your personal subscription.