i have Enrolled my app in Google's (App signing).
so now i Got a :
1: App signing certificate .der
2: Upload certificate .der
so what now ?
how do i use the UploadCert.der to sign APK and Push update ? and what do i do with my old SigningKey.jks that i used to sign my app with !
i thought i should use it in Android studio when i generate APK . so that didn't work.
You should sign your APKs with the keystore you created when generating the upload certificate you uploaded to Play Console.
Note that the .der files that Play Console allow you to download are only the certificates, they don't contain the private key to sign anything. Although you have uploaded the app signing private key, only you has the upload private key. If you can't find the keystore, you can contact Play support team and ask to regenerate a new upload key.