I had already filed with my original W-2 before seeing my employer had also uploaded a W-2C. The only things corrected on the W2-C are the following fields:
Wages, tips, other compensation
social security wages
medicare wages and tips
social security tax withheld
medicare tax withheld
the rest is all blank from boxes 2 (federal income tax withheld - both for "previously reported" and "correct information")
and boxes 7-20 (including all state information is blank).
