Navigate to an asset.
Hover over the asset tile, and click the Folder icon.
From the drop down, select a checkbox for an existing collection.
Navigate to the collection, and you will see that the asset has been added.
There are 2 ways you can remove an asset from a collection.
To remove an asset from a collection via the asset tile:
Hover over the asset tile, and click the Folder icon.
From the drop down, deselect the checkbox for the collection.
To remove an asset from a Collection Summary panel:
Select the collection tile that the asset belongs to. The respective summary panel will load.
You can remove an asset from the collection directly in the Details tab, by clicking the trash icon located to the right of each asset.