You cannot overwrite an existing line. You have to remove it and then to reenter that line.
This is normal when you have allready the same access-list line in your access-list and you reenter the same line again then you will see the error message that you mentioned.
The way to do this would be to remove all lines but let one line in the access-list and reenter the whole access-list again.
If you remove the whole access-list the access-group will also disapear and the access-list will not be active.
sincerely
Patrick