Go to the Payrolls screens.

First, you need to unfinalise the roll and delete the employee's calculation. To unfinalise a roll, first, select the roll from the "Payrolls" screen and click on "Select Roll."

Once in the payroll calculations screen, click the "Unfinalise" icon as highlighted below (bottom right side of the screen) and confirm. 

Terminate the employee's employment with the company as explained in this tutorial and recalculate the payroll. Once ready, finalise the payroll and proceed as usual.