How does one turn off (ie, remove) an app from control center?

See the following from Use and customize Control Center on your iPhone, iPad, and iPod touch - Apple Support

1. Go to Settings > Control Center.
2. Tap Customize Controls.
3. To add controls, tap . To remove controls, tap , then tap Remove.

Hi macdell, have you had a look in Settings -> Control Centre -> Customise Controls?

It may help to know how the app was originally added to the Control Center.