If the LLC is a single-member LLC, then you can just amend the Form 1040 that you previously filed to add the single member LLC.
If the LLC is a multi-member LLC, then you will need to use TurboTax Business (not the same as Home & Business) to prepare a business tax return Form 1065. When you complete the Form 1065, TurboTax Business will create a K-1 that you will need to enter on your personal tax return. Which means, you will still need to amend your originally filed tax return to add the K-1.
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