You would need to go into the database and change all occurrences of
old_field_name to new_field_name
AND
_old_field_name to _new_field_name
If the fields are sub field of a repeater or a flexible content field it becomes more complicated. If they fields are in the options table for options pages or taxonomies, that would be more complicated as well. You’d first need to figure out what you need to change.
This could cause problems if you have other fields in other field groups with the same field name.
