Click on the trash icon next to the asset on the asset listing page.

 

Please note that it is not currently possible to delete/remove individual files within an asset (e.g., an extra set of photos).   As a workaround, you can upload the asset again without the files you wanted removed, and then delete the older asset.  This will result in a new asset with a new URL.

 

Eventually, we plan to have an interface to delete individual files within an asset.  Please follow this page so you can be notified when that is ready.