You can upgrade to OS X Server (Yosemite) to automatically renew your expired push certificates.
If you are using OS X Server (Mavericks) you can use these steps to renew any push notification certificates that have expired:
- Open the OS X Server app.
- Select your server in the Server app sidebar, then click Settings.
- Click the Edit button next to the “Enable Apple push notifications” option.
- Enter your organization's Apple ID in the sheet that appears, then click Renew to renew the expired certificate.
- Deselect (uncheck) the “Enable Apple push notifications” option.
- Select (check) the “Enable Apple push notifications” option.