Generally, for a credit or refund, you must file Form 1040X within. You can change your tax return.If, after using the "search" feature on this webpage, it doesn't return a result, or the notice or letter looks suspicious, contact the IRS at 800.829.1040. Search for Tax Return Preparers. File IRS form 1095-A to get highest federal tax return. Use the format in the sample letter below. Make the letter brief and to the point. Know how to claim your tax refund if you are owed one but didn't receive it.