If it's a Check transaction, you can change the date manually by locating the transaction and editing the date from there. However, if it's a paycheck, the option to edit it is unavailable. As a workaround, you can delete and recreate the paycheck instead. To do this, you can pull up the Paycheck List report to see all the paychecks that you've created.
Select the Payroll tab, then Employees.
Click Paycheck List under Run Payroll,
Change the date range if necessary. Then select the paychecks you want to delete.