If you added your OBi200 to the OBiTalk portal, you must use OBi Expert to make configuration changes. Uncheck both boxes to the right of the value before trying to make the change.
If you want to use the local interface to make configuration changes you need to turn off auto provisioning, otherwise OBiTalk will overlay your change.
System Management -> Auto Provisioning
Under OBiTalk Provisioning
Method: Disabled