For the lighttpd WebGUI web server, follow the below steps:
In System > Advanced > rc.conf,

• set lighttpd_certpem_webgui to NO. This will tell the lighttpd WebGUI instance to ignore the certificate shown in the WebGUI.
• set lighttpd_certpem to the full qualified file name of your certificate, this should be a permanent location which you can update via ssh.
  Whenever the certificate gets updated, the lighttpd service must be restarted as well.

For the lighttpd websrv web server, follow the below steps:
In System > Advanced > rc.conf,

• set websrv_certpem_webgui to NO. This will tell the lighttpd websrv instance to ignore the certificate shown in the WebGUI.
• set websrv_certpem to the full qualified file name of your certificate, this should be a permanent location which you can update via ssh.
  Whenever the certificate gets updated. the websrv service must be restarted as well.