Android Emulator: Installation error: INSTALL_FAILED_VERSION_DOWNGRADE
It means you're trying to install an app with the same packageName as an app that's already installed on the emulator, but the one you're trying to install has a lower versionCode (integer value for your version number).
You might have installed from a separate copy of the code where the version number was higher than the copy you're working with right now. In either case, either:
uninstall the currently installed copy
or open up your phone's Settings > Application Manager to determine the version number for the installed app, and increment your
<manifest android:versionCodeto be higher in the AndroidManifest.