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.