It isn't clear that he can. What's good for Mueller is good for Rosenstein, I believe.
"Anyone advising Trump he has power to remove Mueller better read In re Hennen (1839), reaff’d by CJ Taft (1926) and CJ Roberts (2010), holding that POTUS cannot personally remove an officer appointed by a member of his Cabinet unless an Act of Congress greenlights such removal."
http://twitter.com/tribelaw/status/983867157485416449