RefDB has no special command for a global edit of all references or a group of references. You can get pretty far though with a default file when updating references and with Unix plumbing.
To change existing text:
Run a search on the references that you wish to change and pipe the output through sed into a file, like getref -t ris -d bar -c "sed 's/foo/faa/g' > refs.ris" "search-string".
Use the command updateref refs.ris to update the reference(s) in the database.
To add or change fields:
Run a search on the references that you wish to change, like getref -t ris -o refs.ris "search-string"
Create a defaultfile containing the field(s) that you want to change or add.
Use the command updateref -g defaultfile refs.ris to update the reference(s) in the database.