The Field Type option for a user-defined field cannot be changed after the field has been created. It is important to know ahead of time what kind of data you would like to enter into a field. To change the Field Type, you must inactivate the field and create a new one.
Open the field and uncheck the Active checkbox
Change the display field name to indicate this field is no longer used, and click save.
Add your new field with the correct Field Type. Note the SQL field name cannot be the same as the old field.