How To change quatities in trade contracts (Fearless Blue)

Hello to all

I need help in this topic.

If I have created a Contract with one corporation for like "X" no of qualtity and then after some time if I wat to change that quantity to "Y" what should I do. For this I dont want to cancle a contract but directly want to change only quantity.

Pls tell me if anyone knows....

create another contract for the extra or cancel the existing contract and do a new one for the total quantity