You got this email from Apple and you don't know what to do? Or you saw in the Apple Developer console in the Certificates, Identifiers & Profiles section that this certificate is expiring soon?
It means you need to renew your app Push Notification Certificate. Here are the steps:
Login to Developers account -> Certificates, IDs & Profiles and verify the expiring certificate.
Go to Identifiers -> App IDs -> Click on the AppId -> Scroll down to Push Notifications -> Edit -> Click "Create Certificate" under "Production SSL Certificate".
Create certificate signing request from Keychain Access on your Mac:
Launch Keychain Access -> Choose Keychain Access -> Certificate Assistant -> Request a Certificate from a Certificate Authority.
In the Certificate Assistant dialogue, enter an email address in the User Email Address field.
In the Common Name field, enter a name for the key
Leave the CA Email Address field empty.
Choose “Saved to disk”, click Continue and save it in the computer.
Go back to the Apple developer site and upload the CSR created in the above step -> Continue -> Download the certificate on your Mac.
Double-click on the downloaded cert to install it in Keychain on your Mac.
Open Keychain Access -> Certificates -> The new cert and its private key should be listed there.
Export the certificate for your push notification client: Right-click on the cert in the Keychain Access-> select "Export Apple Push Certificate: " -> leave the password empty -> save as a P12 file.
Login to Firebase -> Project settings -> Cloud Messaging -> APNs Certificates -> Upload your P12 file (Could be you use any other third-party instead of Firebase, find out yourself)