Click on the Federal Taxes tab, then Wages & Income, then I'll Choose What I Work On, then scroll down to Retirement Plans and Social Security.  Select Update next to IRA, 401k, Pension Plan Withdrawals (1099-R) and TurboTax will show you a page with your 1099-R distribution listed.  Select Edit next to the specific 1099-R and you'll be able to change your entries.