To change the opacity of 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 slider on the bottom changes the layer’s opacity - drag it back or forth to adjust.

Note that in the Layer popup, you can find other handy things to do with your layer, like rename it, merge it downward, delete it, duplicate it, lock it, and select the entire layer (the selection arrow).