How to i change the RND amount that I entered when I have not yet filed my taxes?

Return to the 1099-R input section and edit the form ... continue thru all the follow up question screens to make that change. 

To navigate to the follow up screen that will allow you to enter a RMD amount in TurboTax:

1. Sign in to TurboTax and open or continue your return.
2. On the left side menu, select Federal, Wages & Income. 
3.  On the Wages and income screen, scroll down to IRA, 401(k), Pension Plan Withdrawals (1099-R) and select Edit/Add.
4. On the Review your 1099-R info screen, select the 1099-R you want to Review.
5. Continue with the onscreen interview.
6. You'll eventually get to the How much of the $XXX was a RMD? screen. Select one of the options.
7. Enter the RMD amount in the box provided.
8. Continue with the onscreen interview until complete.