For API customization in SugarCRM

Dear All,

I want to customized Some input received in Sugar by  Default API . How can I customized if ? And Also share the path of default API in SugarCRM used.

For Example :-
I have share a case crate API with portal user and I received regularly in any fields 'NA' value , I want to validate in that fields, I did not want to received any 'NA' value. How can I handle this issue from API side. 

Thanks & Regards,
Suraj Kumar