You should be able to navigate back to the Deductions&Credits page, and scroll down to the charity donations section to change them.
Either edit the particular donation....or delete the charity entirely and re-enter the $$ slowly....watching out to not hit the zero key too many times. (Careful with the decimal point too...don't hit the comma)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*