Click on the Federal Taxes tab, then Wages & Income, then click on the "Edit/Update" button next to Retirement Plans & Social Security.
Then you'll land on the summary screen showing Your 1099-R Entries. Select "Edit" next to the applicable 1099-R Payer name and you'll walk thru the screens until you reach the question Any Periodic Payments? and you can make the necessary change. TurboTax will then ask you for the Payment Start Year.
Then continue thru the prompts until you've finished the retirement section.
