Can not update config file for stunnel, they're ready-only

Hi,
Just installed stunnel and want to update its config to server version.

The problem is that I can not change any configs related to it, the topic OpenVPN + Stunnel: Stunnel Listen on wrong port says which files can be updated, but they're read-only and vim can not store the changes.

/etc/config/stunnel
/etc/stunnel/stunnel.conf

Platform: tp-link ac750

Anybody can help me ?

Thanks

Lei