Revision to W2 for box 11

Hello, we have an employee who was no longer at a substantial risk of forfeiture in our company's retirement plan so this year the employee had SERP added as taxable wages for Social Security and Medicare for this year and all prior years. I know I need to include the total amount in boxes 3 (up to SS wage base) and 5. I also need to enter in to box 11 all priors years' deferrals. However I do not know how to force a change to his W2? It is entering the entire amount of the SERP including this year's