Wait until you receive your refund to amend. If you owe, you can amend e-filed returns if they've been accepted and paper-filed returns after they've been mailed.
If your e-file is still pending, you will not be able to make changes until the IRS either accepts or rejects it.
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).
Keep in mind: 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.