If the account owner has enabled this function you are able to trade your shift as follows:
In the weekly schedule click on the shift you want to trade. A new window will open with the details of the shift. Now you click on the button "trade". 

State the reason why you want to trade your shift (only your boss can see this). Before you can finally release you shift for trade, you have to tick the box saying that you are aware that you will stay assigned to the shit until someone else takes over. 

The other employees in that department will be informed about the trade request and can now take over your shift. The two interlocking arrows in a shift symbolize a trade request: 

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