You should add your APNS certs to your existing cert expiry tracking calendar/system. You will also be warned by apple about expiring certs 30 days in advance so whoever is the admin of the Apple Developer account can can add some email rules to forward an alert to highlight the pending expiry (see email example below).
It is good practice to replace the cert 30 days in advance of expiry. Prior to the cert expiring you should create a new cert. To do this go to your apple developer account and find the App with the expiring cert in App IDs and got to edit setting for the App:
Once you get to edit settings scroll down to Push Notifications and you will see you have the option to create another cert:
Create a second cert in parallel to the existing one that is due to expire. You can do this for Development and Production Certs. Creating a cert is exactly the same as described in our docs for integrating the SDK:
Once you have a new cert from Apple you can replace the old cert used by the Xtremepush platform. Log in to your XtremePush dashboard on xtremepush.com go to your app home and navigate to Settings > Certs and Keys and upload the new certificates.
You can return to the Apple developer portal to revoke the Old Cert or you can just leave it to expire in due course.