To which IRS location do you send 941C forms?

Form 941C [Supporting Statement to Correct Information] is no longer used. It has been replaced with Form 941-X [Adjusted Employer's Quarterly Federal Tax Return or Claim for Refund].

Where you file Form 941-X depends on where you are and whether you're including a payment.

If you're in Alabama, Alaska, Arizona, Arkansas, California, Colorado, Florida, Hawaii, Idaho, Iowa, Kansas, Louisiana, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, the Dakotas, Oklahoma, Oregon, Texas, Utah, Washington, or Wyoming, you send it to the Department of the Treasury, Internal Revenue Service Center, Ogden, UT 84201-0005 if not including a payment. If you're including a payment, then you send it to the Internal Revenue Service, P.O. Box 105083, Atlanta, GA 30348-5083.

If you're in any other state, then you send it to the Department of the Treasury, Internal Revenue Service, Cincinnati, OH 45999-0005 if not including a payment. If including a payment, then you send it to the Internal Revenue Service, P.O. Box 804522, Cincinnati, OH 45280-4522.