You do not need to file an amended return. Code C in Box 12 is Taxable cost of group-term life insurance over
$50,000. That amount is included in your Box 1 wages. Adding the code to your return won't change your return. Moreover, the information is reported to IRS by your employer. There is a remote possibility the IRS will contact you by mail regarding this. If so, you will be able to respond by mail and resolve it.
