Hi Sreekanth,
You cannot change the baseline directly, that includes the API.
You can save the baseline from the Clarizen UI or through customization methods (e.g. workflow rule), which in turn can be triggered by the API.
You didn't elaborate why you need to change the baseline values.
Perhaps you could use custom fields for those changeable start/due dates. Custom fields can be 'API updatable only'.
Hope this helps,
Ophir