Hello,
You can achieve this by doing the following steps:
1. Go To WCFE Editor Dashboard Page
2. Click Preview button
3. Manually update wp-config.php file and save
However WCFE Plugin is currently able to work with pre-defined fields only so that every time you want to update wp-config file you will need to manually rewrite those again.
Thanks for the question. It helps me find two more other features I will added to the next version. In next version I will allow it to support custom fields so that you will just define them only one time and then change their values whenever you need.
Thanks,
AHMeD