Download the Form MO-1040 for the year of the tax return you are amending from the DOR website or obtain one from your local public library. Obtain a copy of Form MO-A if you itemized your deductions.
Make a copy of the Form MO-1040 you originally filed, your federal tax return and all other supporting documentation such as your W-2 or 1099.
Complete Form MO-1040 and Form MO-A, if applicable, using the correct figures. Complete the "Amended Return" section on page two of Form MO-1040 and add the interest and penalties to the amount you owe.
Mail your amended tax return, copies of your original tax return, copies of your federal amended tax return (Form 1040X), and all supporting documentation and a check or money order for any additional amounts owed to:
Missouri Department of Revenue PO Box 329 Jefferson City, MO 65105-0329
If you are due a refund, mail to:
Missouri Department of Revenue P.O. Box 500 Jefferson City, MO 65106-0500