First of all - this is almost certainly not a good idea unless it has been confirmed by the office, or was a manual check-in with no geolocation checks. If you still want to go ahead...
Click on the check-in information button for the care worker whose check-in/outs you want to reverse. 
(maybe only the check-in arrow on the left is there - if that is the case skip the next paragraph)
On the row that contains the Finish information, at the far right hand side there is an almost invisible light grey "x". Click on the button and you will be asked why you want to revert the Event Finish. Do so and click on OK.
On the row that contains the Start information, at the far right hand side there is an almost invisible light grey "x". Click on the button and you will be asked why you want to revert the Event Start. Do so and click on OK.