If you have an expired certificate error, this is because iOS certificates expire after one year.
To fix this issue, login to https://developer.apple.com/account and re-create any expired certificates. You are basically re-creating your .p12 and provisioning profiles explained here from scratch. If we created your certificates, email us for help.
Once you have your new certificates, go to your myapppresser.com dashboard and click on your app. Next click push notifications in the left sidebar.