When a user submits a modification to one or more parts of any record on Ancestry.com, the original index information is not changed - it is added to. Another entry is put into the database with the modified entry ("updated") submitted by the user.
Here is the process I used yesterday to modify the surname of one of my families that were incorrectly indexed in the 1870 U.S. census. I looked for my great-grandfather, born Henry Austin Carringer in 1853 in PA, and I knew that he lived in Iowa in 1870. He should be with his parents David J. and Rebecca Carringer and his brother Harvey Edgar Carringer. Finding him was a challenge because of how his name was indexed, but I eventually found him in the 1870 census in Washington County, Iowa indexed as Henry A. Current. Here is the screen shot of the census record:
As you can see, the name is difficult to read - the enumerator's scrawl is almost as bad as mine). I can understand how it was indexed as "Current" and not "Curruntr" or similar. I can almost see the letter "g" in the surname but it's hidden by Rebecca's "b". But I absolutely KNOW that the surname is "Carringer" based on other records, so I want to UPDATE the index for all five members of the family.
Henry's name is highlighted in the index, so I clicked on the "Add Update" button on the left side of his name:
The popup box above offers to change either the Family Number, the Given Name, the Surname, the Estimated Birth Year or the Birth Place. I chose the Surname. Once I did that, another field appeared in the popup box and I had to choose between Transcription Error, Incorrect in Image, Nick Name, Maiden Name, Name Change or Variation.
I chose "Transcription Error" (even though it probably wasn't - now that I look carefully at the image, it should have been an "Incorrect in Image" reason). Once I selected "Transcription Error" then I could add the "correct" Surname to the bottom field in the popup box:
Clicking on the orange "Close" button completed this "Update." I proceeded to do the same exercise for the rest of the D.J. Carringer (indexed as "Current" family - all five of them (daughter Effie appears below the edge of the screen below) and saw:
So I have successfully updated the surname for this family in the 1870 US Census. If the given names, birth year or birth place was erroneous, I could have updated them also. All of the other data is OK with me for this family.