Postgres 10

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table services (properties text);

CREATE TABLE

INSERT INTO services (properties) VALUES ('{"url" : "www.example.com"}');

INSERT 0 1

UPDATE 
   services SET properties = jsonb_set(properties::jsonb, '{url}' , '"www.mydomain.com"' )::text
WHERE 
   properties::json->>'url' = 'www.example.com';

UPDATE 1

select * from services;

properties

{"url": "www.mydomain.com"}

SELECT 1