I got the updated code and apk from the app developer. I have rebuild the app with my own keystore files and signed the app and uploaded to the store and received "uploaded APK is signed with different certificate to your previous APK".
I checked the updated apk sha1 files by
jarsigner -verify -verbose -certs updated_apk.apk
and the sha1 files showing that are of the developer and the apk is not signed with my keystore files.
How to sign the updated app with my own keystore files?