Use the Net to Gross Calculator from the Manage Payroll>> Process Payroll>> Record Gross Pay section.
Please refer below screenshot for more help.
- Please note that the status needs to be pending of the client.
- Net to gross will work only for the employees who are on variable pay
Once a desired net pay is entered in the box, the gross pay will be calculated 
Once the save button is pressed the updated gross pay will be reflected in the payroll