I have android app (I lost its source code), after a month I have found i wrote something wrong in the application and I need to update it or send a notification to users. so I rebuilt the app and tried to upload it again as update. but it is rejected because it doesn't have the same private key.
Is there any way to retrieve the private key or send notifications for users??
The only way to update an app is by building it using the same package name and keystore. It's called a keystore for a reason - it's your key to the app and nobody else can get at it. If you've lost it, there's nothing that can be done - even Google wouldn't be able to restore it for you. If you publish on Google Play, it's essential to keep a backup of your keystore to avoid this issue.