How to change cascade setting for opportunity to salesorder in CRM 2013?

Hi,

I want to change cascade setting for opportunity to salesorder in crm 2013.

i have gone through the path settings->customizations->cutsomize the system->entities->opportunity->(1->N relationship) and the screen shot is as per below.

where if want change the type of behaviour to customize i can change the setting.

But, here it is readonly setting for this relationship and type of behaviour is "system".so, i was anable to change the setting.

Can you please let me know how to change settings for that Cascade All to Cascade None.

Thanks,

Kandarp