While standard SAP does not provide a tool to Mass change the contents (fields) of IDOC, there are other custom products/ utilities out there that are very flexible, user friendly, and not only provide Mass Change functionality but also provide many other features.
One of a very comprehensive product is Error Message Manager (EMM). Details can be found at www.omksolutions.com Or request additional details by sending an email atemm@omksolutions.com
Thanks,
Prashant