Hi jaya,
if you want to change the contact person that is determined. There is some functional setting regarding the partner determination where you can make partner function as grayed out or changeable to your transaction. there are following steps that you need to follow.
1. Determine the Transaction type of Transaction.
2. Go to SPRO setting chose Transaction type then double click on it to determine the which partner determination is defined there.
Determine the Partner Determination Pr.
3. Then Go to Basic function->partner processing ->
find out the partner determination and click on partner function in procedure there you will know which partner function is allowed to change or not or how many partner function of same type you can maintain in transaction such two contact person .
if you want to make contact person changeable then click the check box and you will be able to change the contact person or any other partner function which ever you want.
If this is helpful then reward with point.