Garmin Connect won't install or reinstall app
Power your Garmin off and turn it back on. It may be a cliché with electronic device, but this fixes all sorts of problems with the Garmin installation process!
Garmin installer says my device firmware is not up to date
Try disconnecting your watch, and you may find that it will then install and restart with updated firmware. Then reconnect and install the app again. You may need to try this a few times. Also try powering your watch off completely and restarting it.