Hi Srilakshimi,
If you are familiar with MASS transaction, then you can modify User Responsible field for Internal Orders from transaction KOK2.
As first step you must create a selection variant in order to define which orders you want to modify. Once selection variant was created, excute transaction with it and you'll get a screen similar to MASS transaction. Select the field you want and massively replace it. Do not forget to save.
Best Regards!
Mgitur