sales order changes at header leavel

Hi experts,

If there are for example 5 items in sales order and all items date is to be changed where at header level we can amend so that that's applicable for all items at item level.

10 Mat1 Delivery date 09.05.2012

20 Mat2 Delivery date 09.05.2012

30 Mat3 Delivery date 09.05.2012

40 Mat4 Delivery date 09.05.2012

50 Mat5 Delivery date 09.05.2012

Now if i want to change the Del date to 20.05.2010, instead of changing at each item level where can i change this date at header level so that I dont have to amend at each item level.

Replies will be appreciated.

Thank you

Abhi