I forgot to add untaxed income to W-2. How do I fix this?

You will need to file an amended return and add Form 4137, Social Security and Medicare Tax on Unreported Tip Income, to report the amount of your unreported tip income as additional wages. You will owe the employee share of social security and Medicare tax owed on those tips.

Your employer is not handling your tips correctly.  According to the IRS:

"Employers are required to withhold taxes (including income taxes and the employee’s share of social security tax and Medicare tax) based upon wages and tip income received by the employee and to deposit this tax.  In addition, employers are required to pay the employer share of social security and Medicare taxes based on the total wages paid to tipped employees as well as the reported tip income. This information and tax is finally reported to the IRS on the appropriate forms by the employer.  You will owe the employee share of social security and Medicare tax owed on those tips.

Tips reported to the employer by the employee must be included in Box 1 (Wages, tips, other compensation), Box 5 (Medicare wages and tips), and Box 7 (Social security tips) of the employee's Form W-2, Wage and Tax Statement. Enter the amount of any uncollected social security tax and Medicare tax in Box 12 of Form W-2."