Once you have swiped left on the desired task and are viewing the three options: Completed, Edit & Delete, tap the left purple button that reads ‘Completed’. A translucent white box will pop up that reads ‘Amount Completed’ with a number selector. Below that will appear, from left to right, a negative number (Selecting this will decrease the amount completed.), Due (This option will complete the amount due today.), and a positive number, which will increase the amount completed. Once you are done, press ‘Select’.
How do I update the amount a task has been completed?