How can I restore a Whatsapp backup without uninstalling it (Whatsapp is a system app)? Could I just go to app settings and click clear data?
If you have a WhatsApp backup in Google drive or a local backup, and you'd like to restore it, all you have to do is clear data for WhatsApp. To do that, go to settings &rarr; apps &rarr; WhatsApp &rarr; storage &rarr; clear data.

Now, when you open WhatsApp, it well be as if you have just download the app. It will ask to verify your phone number, and it will then give you the option to restore your chats.