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From the Box website, click on the All Files dropdown () menu and then click on Trash. Alternatively, click on your Name at the top right and then click on Trash. 
You'll then see a list of everything that's been deleted. You may use the checkboxes (on the right side) to select multiple items and then click on the Restore button to restore the files or folders to their original location.

or

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Mac

The Box Sync Folder for Mac OS users will have links to your Box Sync folder in Favorites as well as the right hand side of the Finder.

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