Federal and State Tax Amendment Instructions Tax Amendment links with access to forms, instructions etc. How to File an IRS Tax Amendment Via Form 1040-X Online See Instructions on How to Complete a Tax Return Amendment Form 1040X or 1040-X. Amend State Returns; Amend Back Taxes; Prepare Online and Mail. Amendment 1040-X for Your Latest Tax Return - Prepare Online Tax Amendment Form 1040-X: Prepare 1040-X on eFile.com and Print, Sign, and Mail It to The IRS. File Your Taxes and Amendments on eFile.com. Tax Amendments by State: Fill Out Your Amendment Online Tax Amendment Forms by States Can Be Found Here. See Detailed Instructions on How to File a State Tax Amendment and Mail It In. TurboTax® is a registered trademark of Intuit, Inc. H&R Block® is a registered trademark of HRB Innovations, Inc.