If you don't care about other headers like the transport info then it's easy.
Drag the message to the outbox (that's the only folder that allows editing of messages), edit the header save it (Don't send!) and drag it back to the folder of origin.

In case you want to keep the original tansport info, time/date of creation, message-id and so on. Then you'll need to export it (preferrably to unix mailbox format as that remembers the original received dat), edit it with a third party tool (notepad would suffice) and finally import it back.
In the macro-library at the TB-faq you'll find an example of a manual filter that automatically takes care of the exporting, importing and starting a (plain) text editor: http://cgi.silverstones.com/library.php#mssg__header_editor