I have released an app on Google play store. Now I want to change the private key and release a new update for that app and I have previous used private key, I want to replace it. Is that possible? please help me. Thanks in advance... :)
You cannot change key used to sign released app. The only way is to unpublish current app and release new one (with different key and different packageId -> so no updates to old app users):
Warning: Keep your keystore and private key in a safe and secure place, and ensure that you have secure backups of them. If you publish an app to Google Play and then lose the key with which you signed your app, you will not be able to publish any updates to your app, since you must always sign all versions of your app with the same key.
https://developer.android.com/tools/publishing/app-signing.html