From what I can tell, there is no simple way to do this.
One thing you can do is to create Workflows that set the Task outcome to "Approved" or "Rejected" or what have you, and then link that to a Custom Action defined on the list Display form. Then you get a "one click" solution to updating the form- however the custom action buttons appear in the Ribbon by default and can't easily be moved into the form itself.
I have tried both of the above but neither is ideal because manually initiating workflows require elevated permissions on the list item.
The below solution seems like the simplest, even though it is "not smart". Essentially you create a custom Display Form for the Task list that immediately redirects to the Edit form of the same list.
Automatically open SharePoint 2013 workflow tasks in Edit mode for easy one-click approvals