The problem is the keys that have been used to sign the APKs, by default if you are running directly from your IDE and opening your Emulator, the APK installed in the Emulator is signed with your debug-key(usually installed in ~/.android/debug.keystore), so if the previous APK was signed with a different key other than the one you are currently using you will always get the signatures conflict, in order to fix it, make sure you are using the very same key to sign both APKs, even if the previous APK was signed with a debug-key from another SDK, the keys will definitely be different.
Also if you don't know exactly what key was used before to sign the apk and yet you want to install the new version of your app, you can just uninstall the previous application and reinstall the new one.
Hope this Helps...
Regards!