When you're in your return, click on Federal Taxes (the 2nd tab at the top), then Wages and Income (the 1st button), then scroll down to Retirement Plans and Social Security and click on the Update button next to the line that has (1099-R) on it. Then click the Edit button next to the "Paid By" entry you want to fix. If it's a joint return, TurboTax asks who's 1099-R it is, then click continue. Scroll down to Box 7 and fix your error by clicking the drop-down arrow and picking the correct number or letter or even picking (No Entry).
