You can change your employees contribution you have setup earlier. To edit employee rate of contribution, go to Employee button in the Tasking zone. Please follow the below Road Map:

Road Map - Change employee rate of contribution
Employee -> View/Edit Employee Details -> View/Edit on selected employee ->(Employee Name) AE Pension -> Edit ->Employee Contribution -> Save
( 'Employee' is the button positioned in the tasking zone menu at the top. It is the second button from the left)