Before you delete a folder and everything in it, remember that assets can be in multiple folders at once in Dash. So make sure you really do want to delete all the assets before you go any further!
To delete all the assets in a folder:
Open the filter panel and select the folder:
Select one of the assets in the panel, and then click "Select all X" in the selection bar at the top to select all the assets in the folder:
Click the Bin icon in the top right, and then confirm:
You can delete the folder itself from Admin->Folders or by clicking the 3 dots next to the folder in the Filter panel. For more on this How do I manage folders?