How to create signed apk using upload_cert.der file

I have read many reference about Google app signing but still did not found my requirement. I have only upload_cert.der and deployment_cert.der file. I have not access of publisher play store account. I have to only created updated APK and deliver its to client and client will upload it. Now my question is that :

• If i have only upload_cert.der and deployment_cert.der file then, can we create signed apk which can update existing application on store?
• If above requirement is possible then please explain step by step process of creating signed apk using upload_cert.der and deployment_cert.der file that can successfully update exiting application.