You can search on the keyword, select all search results, then go to the Keywords dialog and delete/uncheck the old keyword, and add the new keyword.
You could also use
SQLite:
BEGIN;
UPDATE UserKeyword SET Keyword='newword' WHERE Keyword LIKE 'oldword';
UPDATE ItemKeyword SET Keyword='newword' WHERE Keyword LIKE 'oldword' AND AttributeID=1;
COMMIT;