How do I include a letter from my employer with my tax return that is titled "409A Relief Under V.B and V.D of Notice 2008-113"?

I received a letter from my employer that they said must be attached to my 2024 federal income tax return, which is titled "409A Relief Under V.B and V.D of Notice 2008-113". I already filed my 2024 taxes, and will need to file an amended return. How do I file an amended return with the only change being a letter from an employer?

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