Un-Picking from the classic admin console is done by navigating to the the transaction from the home page or Search>Transactions.  

Once you open the Sales Order, you will click on "Detail" to the right of the document line that you would like to Unpick.  

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A window will be displayed with the details about the line.  Click "Delete".

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A window will appear asking you to verify the unpick action.  Click "OK".

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The inventory is now Un-picked and will be returned to the Bin from which it was picked.