As always, It Depends....
 
The payroll W2 file is a copy of the payroll master file as of year end.  You can copy BKPRMSTR (from a year end backup for your company after the last payroll of the year was run) to BKPRW2 (your company) to fix the W-2's for last year but you also have the issue of  the current year being incorrect.   That is trickier.  If you don't have many employees, PR-A and edit the YTD and QTD amounts.  If you have mpre employees than you want to edit, email me, I can walk you through it.