MY ss # was off one digit when I first was hired, so my first 3 months of work had a different ss # which I reported to my employer after finding out. They changed the number correctly but now I recvd my W2 and the amount for the federal withholding is incorrect, s/b around $400 but they put about $192, what should I do?

Ask them to issue you a corrected W2

Yes, that is correct. Go to your payroll office and point out the error and ask them to issue a corrected W-2.