Sometimes you need to adjust the hourly accounts of an employee and this is how you do it.
Go to edit employee and start by making sure that the hired date and the hours per week are filled properly.
Then scroll to "Hourly accounts regulation date" and enter the date from when the adjusted number of hours should apply.
In the "Hourly accounts regulation hours" field below enter the number of hours that the employee accounts should be on the beginning of the above-entered date.
Here's a video showing you how: