If you have an employer provided 401(k) retirement account and you are still employed by that employer your retirement contribution is on your W-2 in box 12 with a code of D. There is no need to amend your return since it has already been reported.
If you are referring to contributions to an IRA then you will have to amend the 2021 tax return to add the contribution.
See this TurboTax support FAQ for amending a 2021 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-2021-return/L1gEU4G3X_U...
