Did you know you can select and adjust an entire layer at a time? To select a layer:

1. Tap the Layers menu to open it.
2. Tap the layer you want to change (it will become highlighted in gray).
3. Tap your selected layer (again) to open the layer popup.
4. The arrow on the left side of the popup is a selection arrow (just like the selection tool appears as an arrow). Tap it to select your entire layer, and make the adjustments as you need.