To delete everything on your canvas at once, double-tap on a Mask or Slice tool in the tool wheel. You’ll be given a menu with the following options: Clear Current Layer, Clear All Layers, Clear All Strokes, Clear All Text, and Clear All Images. Select the option you want, then tap Confirm.
To quickly delete elements of your drawing, Select a stroke or multiple strokes, then tap the Delete icon on the Selection popup.
To delete everything on a Layer, tap a layer to activate it, tap it again to bring up the layer's menu, then tap the Delete icon.