This error occurs when you try to reinstall an android application that is signed with a different key than already installed application. This may occur if you are a developer and installed the signed version of the application to your device (from Google Play Store) and you want to install a debug version of the same application for development purposes. To prevent this you may sign your development/debug versions with the same key that you use for publishing your application to Google Play Store. To do this you have to change your debug certificate with your real certificate. Debug certificate used is stored in the .android/debug.keystore file in your home directory. You have two options:
I will describe steps for second option.
keytool –storepasswd –new android –keystore debug.keystore
keytool –list –keystore debug.keystore
Above command will list key aliases and their corresponding certificate fingerprints. Find your key alias you have used to sign your application for Google Play Store
keytool –keypasswd –alias your_key_alias –new android –keystore debug.keystore
keytool –changealias –alias your_key_alias –keystore debug.keystore
Above command will ask you destination alias name, enter androiddebugkey. It will also ask your store password like all above commands.
After that your debug versions can be installed over the version downloaded from Google Play Store.