From the Payroll Calculations screen select the employee you would like to change the number of contributions for. From the payslip section click on the "Edit" icon next to Social Security.
In the Social Security window select the SSC you would like to edit, and click the "Edit" icon located in the header menu. (If you cannot click the Edit icon, you may need to unfinalise the payroll, by clicking "Unfinalise" at the bottom right side of the screen.)
Now you can amend accordingly.