You have three years from the date you filed your return or two years after you paid the tax due (whichever is later) to file an amendment.
Select the tax year you're amending for specific instructions.
Wait until you receive your refund to start your amended return. If you owe, you can amend e-filed returns once they've been accepted and paper-filed returns after they've been mailed.
Once you begin the amendment process, you'll see your original return. Only make changes to the areas of your return that need amending. The refund calculator will start new at $0 and only reflect the changes in the refund (or tax due).