In T Code MASS please select Sales Orders in object type and execute.
Then you choose the relevant tables and the fields which you want to change and then execute it.
There is a Transaction MASS which can be used to carry out mass changes in the sales order.
Thereafter you enter the document range and also the relevant datas and then you choose "Selection Field". Add the fields from Pool to Selection Field using the arrows.
Now you go to Mass Maintenance menu and click on "Enter New Field Values".
Choose the data and then you execute it . Thereafter you will see that the datas have changed and then you need to save it .
Once it has been saved, the values will change.
You can check the status through SM37.
You can alternatively go to SE16 and go the particular table. Then do mass maintenance and compare the results. If they match then it has been done