There is no procedure on your federal tax return to claim a refund of excess Social Security or Medicare withholding if it was due to the mistake of a single employer. (there is a procedure if you work for more than one employer and the total withholding was more than the annual maximum, but not if this was a mistake of a single employer.)
your employer must fix this. The employer must correct your W-2, refund the excess withholding, and also correct their own quarterly tax statements.
If your employer refuses to fix the mistake, you can try to get a refund of the excess by filing form 843. This form is not supported by TurboTax and you will have to do it manually. You will need a letter from your employer acknowledging the mistake and in which the employer states that they will not be making the adjustment themselves.
