I have already filed my 2020 taxes on a 1040 tax form. I need to amend them to add an 401k inheritance to my taxes. How do I do that?

Generally you would not need to report this unless you received a 1099-R.  If you received a 1099-R you would need to amend to report for that form. 

Usually your inheritance is not taxable and is not reported on your tax return. However if you inherit property that produces income such as interest, dividends, or rents, that income is taxable and reportable on your return

Most often, distributions from an inherited 401(k) are included in a beneficiary's regular taxable income.  If you inherit a Roth 401(k), distributions may be tax-free if your parent first began making contributions to their "designated Roth account" at least five years before you begin your own withdrawals.

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Is an inherited IRA or retirement plan taxable?

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