Update a json element in json datatype
Since it's just a string, you might be able to accomplish a simple change/deletion of a node with the regex_replace function.

For example, this is how I recently deleted a certain JSON node in a table (all rows):

UPDATE my_db.my_table
SET my_column = (regexp_replace(my_column::text, ',"some_json_node":(.*),', ','))::json
WHERE NOT my_column IS NULL


Note, for all my JSON data, I keep a "version":"(n).(n)" (i.e. schema version) node in the object. That way I can update objects that comply with a specific version. Your requirements may not be that complex, but if they are, it certainly helps.