RefDB has no built-in reference editor (but see the Emacs and vim support files which allow you to add and update references from within your editor). The general strategy to edit an existing reference is as follows:
Save the reference(s) that you wish to edit to a file using a command like getref -o foo.ris -t ris -d bar ":ID:=1717 OR :AU:=^Miller". This would retrieve the reference with the ID 1717 as well as all references with a Mr. or Mrs. Miller on the author list. If you prefer, use the risx output format instead.
Use your favourite text editor to edit the references in the file foo.ris. Make sure you don't touch the ID field as this is crucial to match this reference in the next step with the existing copy in the database.
Use the command updateref -d bar foo.ris to update the reference(s) in the database.