First, change your form definition in CF7 so that all future form submissions have the field names you want.
Second, update the database to change the old form entries to the name that you want. If you purchased the editor, you can simply edit the column name.
Otherwise, you will need to connect to PhpMyAdmin and execute a query to change the field names. For example, if you have a field named “menu-684” that you want to be called “country” in a form called “Contact form 1” you would use a query like the following:
update wp_cf7dbplugin_submits
set field_name='country'
where
field_name='menu-684'
and form_name='Contact form 1'