Did you receive an adjusted Form 1095-A from the marketplace? Or, did you get a letter from the IRS requesting Form 8962?
If you received the adjusted Form 1095-A - then you will need to amend your tax return. You can follow the general steps in the article below. When you get to the steps where it lists items to change, Form 1095-A will be one of the options and you can follow the prompts. Do note that the IRS recommends that you wait until you have received your first refund before you amend.