How can I deauthroize the Facebook app?

You should go to the Facebook Settings which you can reach from the top menu.
Facebook Settings

You’ll taken to the General settings tab, but you’ll need to go to the Apps menu.
Facebook General Settings
This page will lists all of your apps. Find your app by its name. Move the cursor over the app’s block and hit the appearing Remove button.
Remove Facebook App

After this when you try to log in with Nextend Social Login again you’ll be asked to authorize the app again.
Facebook App Authorize