As long as there are no outstanding T&Cs or Developer Licence Agreements needing to be accepted, your push notification certificate will automatically be updated by Digistorm around 14 days prior to its expiry date.
To check for outstanding requirements, log into your Apple Developer Account. Note that if your school uses Schoolbox to send push notifications, you'll also need to request a copy of the new push notification certificate to replace the expired certificate in your Schoolbox instance.
For more information, please contact Digistorm Support: support@digistorm.com