how to use touch id to update an app ?

Hi

In settings under touch id and passcode check that you have use touch id for itunes and app store enabled.

Also check in settings, itunes and app store whether you have automatic downloads selected for updates if you do this bypasses the entering a password or touch id each time.

In iOS if you are using the Update list in the App Store app then you will not be prompted for a passcode or touch ID. There is no way to force those listed updates for installed apps to ask for a password or touch ID. The settings available to you in iOS affect installing new apps, not updating apps already installed on the device.