Hi
Use se16n EDIT Option and change the Quantity in - to +ve value in VBAP and VBEP.You can refer below link how to use se16n .
Once you have executed se16n system will allow you to come and check individual / Collective sales Order .IF required do the necessary correction in sales Order individually or collectively .
SE16N and SAP_EDIT « Sap Security Pages
Regards
Santosh Verma