Text below from Android Authority link:
The device comes with a Storage Wizard, that lets you uninstall apps that you haven’t used in a while, and delete files that you no longer need or have backed up. You can find this by going into Settings – Storage – Make more space.
Some third-party apps can be moved from the phone storage to the storage card. Go to Settings – Apps – On SD card, to see which apps can be moved. To move, tap an app, and then tap Move.
Back up files, photos, and videos to your laptop or to the cloud and keep only the recent and more important documents on the device.
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/devicehelp.optus.com.au/htc/desire-816/troubleshooting/entertainment-and-internet/i-can-t-install-an-app/
http
/www.androidauthority.com/htc-desire-816-problems-531456/
https/www.quora.com/How-can-I-move-install-the-applications-on-the-HTC-816-Desire-without-rooting-it
https/www.youtube.com/watch?v=vCjHzM3me4c