If your IRS letter does not specifically state that you need to Amend your 2018 return, they may not need you to do this (or amend your 2019 return).
You could use the Contact Info provided in your letter to verify this.
Yes, you could open your 2018 return as if you were amending to check their calculations, although they often send you their calculations with the letter if tax is due.
Click this link for info on what to do if You Received an IRS Notice.
This link gives instructions on How to Amend a Prior Year Return.
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