How to upgrade Aerogear ag-push from 1.3.1 Final to 2.5 latest

I have already existing setup of Aerogear Ag-push 1.3.1 final. Several projects I have added. Now I'm not able to send notifications (since last several months its been down). I'm getting following errors:

Variant XXX has failed: Error sending payload to APNs server: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_expired

Variant XXX has failed: Error sending payload to GCM server: HTTP Status Code: 301()

Variant XXX has failed: Error sending payload to APNs server: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_expired

What can be the reason? also I would want to retain my projects/users and upgrade ag-push to 2.5. How to do an upgrade? I am not able to find any instructions.