Empty downloads folder

1. In the corner of your screen, select the Launcher   Up arrow .
2. Open Files .
3. Click on the Downloads folder
4. Drag files you want to keep to MY DRIVE
5. CTRL+A to select all remaining files
6. Select Delete   Delete.

Note: Files in the Recent, Images, Videos, and Audio folders are viewable only and can’t be deleted directly in those folders.

Source: https://support.google.com/chromebook/answer/1700055?hl=en