Please try the method below to flip a layer horizontally or vertically.
Tap the layer icon on the drawing screen.
The layer settings page will open. Tap on the gear icon of the layer you want to flip.
Tap on "変形（Transform）". 
Grid lines will appear and they'll allow you to select a range and size. Please specify the appropriate size.
You can flip the layer horizontally and vertically by tapping the triangle icon at the bottom of the screen.
Tap "決定（Done）" to finish.