You can select a single asset, multiple assets or all assets at once.

1. Tab on Select.
2. Tab on the Checkbox in the upper right corner of an asset's preview to select it.
   Note: If you are in the list view the checkbox will be located to the left of the preview.
   The box is highlighted in orange if the asset is selected.
   
3. Make your selection or tap Select All. 

At the top you will see how many items are currently selected.