If you are talking about DATABASE-Fieldkeys, then you can’t change them anymore, once you have created them (correct me if i am wrong).
You will have to recreate a new FIELD(FIELD-KEY). That means → remove the old → COLUM and adding/creating a new one with your wished new FIELD-KEY.
This is why i am also first developing my database as good as possible, before i start → coding!
DATABASE = Most important part of your whole project !!!
A site without a DATABASE, is not a real site!