To rate an an asset, select one or more assets on the thumbnail grid page (tip: use Shift and Ctrl-keys to make multi-file selections) and then click on the Set Rating button in the Action bar. Finally, click on a star rating in the overlay to apply it to the selection of files.
Rating assets from the Details page
When viewing an asset on the Details page you can rate it by clicking on the Action button and then choosing Set Rating. Then choose a star rating in the overlay dialog.
Clearing the rating
To completely remove all starts from one or more assets, select them in the grid and choose Set rating from the Action bar. Then set the rating to 0 stars.
The same can be accomplished on the asset Details page by clicking on the Action button above the preview and choosing Set Rating.