To renew a customer, open your reseller panel, find the line you want to keep going, and choose Renew or Extend. The existing line stays exactly as it is and the new time is simply added on top.
Nothing is reset when you extend. The customer keeps the same username, password, and server, so they do not need to log in again or reinstall anything. Their app keeps working without interruption.
Renewing costs credits the same way creating a line does, based on the length and number of connections you add. This is the smoothest option for repeat customers, since there is no new setup on their end.