There are three different ways you can delete an asset.
A. Normal View
Click the checkbox next to the asset(s) that you want to delete. A set of icons will pop-up directly above the list of assets. Look for the trashcan icon and click it. A prompt will ask you to unpublish the asset. Leave the checkbox checked and click Confirm Delete.
B. From the "More" Menu
Select the asset you want to delete. Click More at the top-right of the screen and then choose Delete. A prompt will ask you to unpublish the asset. Leave the checkbox checked and click Delete.
C. Right-Click
Right-click the asset you want to delete. Click Delete from the dropdown menu. A prompt will ask you to unpublish the asset. Leave the checkboxes checked and click Delete.