You can see a list of tasks you completed on that day in your day view (at the very bottom).

So if you just marked a task done it will show up in the "completed tasks" section for that day. 

Unmarking it done

You can then click on the checkbox icon again to unmark it done.

Alternative using the archive search

If you don't see the task there, you can also go to the archive (menu at the top right) and then click the magnifying glass icon there.

Then you can search for the completed task and uncheck it there by clicking the checkbox.

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