Using your Android file browser, navigate to the folder where your apk file was downloaded. Click on the apk file (in this case, named HandHygiene_google_play).
Blocked installation
Android will prompt you with a warning that your phone does not allow installation of apps that do not originate from the Google PLAY store.
Click on "SETTINGS" to open the security settings.
This will allow you to modify permissions, to install the downloaded app.
Changing your installation settings
Tap on the checkbox for "Unknown sources".
Changing your installation settings
Read the warning dialogue, ensure the checkbox for "Allow this installation only" is selected, and then tap "OK".