Android Question You uploaded an APK that is not signed with the upload certificate

I have a new PC, have used my original keystore file and when I tried to upload a new app version I get the following error

You uploaded an APK that is not signed with the upload certificate. You must use the same certificate. The upload certificate has fingerprint:SHA1: DD:2B:8F:AB:67:57:7C:E5:57:129:88:1D:EB:A7:E7:6D:7B:8D:F5
and the certificate used to sign the APK that you uploaded has fingerprint:SHA1: 58:1D:66:84:240:E3:E4:59:4B:2F:A7:5B:45:60:97:1E:20:77:F0

I have seen the post https://www.b4x.com/android/forum/threads/google-play-app-signing.91703/ which makes no sense to me, as I cannot find a JAR file to download on play store, only the deployment_cert.der and upload_cert.der

Can someone point me in the right direction, as I desperately need to update my app.

Thanks,
Kev