If you want to delete your lead because it converted into a user, you can achieve it by deleting it in your backend as soon as he/she subscribe on your platform.
If you want to transfer all your lead properties you need to ask for the lead info from your backed before deleting it and post the user.
First thing first, you need to get the lead info with a GET on /v1/leads endpoint.
Now be sure you store the returned payload to send it back later when you will create the user.
Once you have got the payload with the lead info, you can now perform the DELETE on /v1/leads.
Now that you have successfully deleted the lead, you can proceed to create the new user via a POST on /v1/leads/
You have successfully converted a lead into a user 🙂