Tap 'Launch'. A dialog box to download .apk file will open on your device, tap Ok or Download

Open the downloaded .apk file, and tap 'Install'
(Please note if you haven’t enabled 'Unknown sources', a dialog box asking to enable 'Unknown Sources' will popup. Go to settings and enable it.)

Once the app is installed, you can delete the .apk file downloaded earlier as you wouldn't need it any longer.