You can rename a payroll field by going to the Maintain menu > Payroll > Reclassify And Rename Payroll Fields. However, renaming the field doesn't update the payroll formulas that refer to that field name. You have to do that manually by going to File menu > Payroll Formulas > User Maintained.

If your Sick/PTO is accrued on each check (not lump sum method) you'll need to edit both the SICK_ADD 23 formula and SICK_REM 23 so they refer to the new field names. For example, the SICK_REM formula would change from ANSWER=SICK_Accrue-SICK_Taken to ANSWER=PTO_Accrue-PTO_Taken. (or whatever you renamed the fields to). It doesn't matter that the formula name and ID still use "SICK" as long as the formula refers to the correct field names.