Once you have located the animal you would like to mark as deceased:
Click Mark as deceased from the top-right hand side of the screen, above the measurements graph.
You can then choose the deceased date, provide a reason and add any comments you would like to include.
This animal will then appear in the Deceased section at the very bottom of the screen in the Cages column.
If you wish to undo this, choose the animal, click Actions and then Undo. You can then choose which cage the animal is in.