Calculated Column Formulas

I need to change the formula in a calculated column. I entered the new formula that I want to use in the first row and copied it all the way down. Now I have a triangle in the upper left telling me "This cell is inconsistent with the column formula". How can I change the column formula to the new one so I don't get this error? I cannot ignore it, as I know is going to be suggested, because I have new rows added to my calculated column and it automatically inserts the old, incorrect formula.
So, how do you change the default column formula?

Hi and welcome on board! Would you please post the sample for us to look at?