MikeBOS gives almost excellent advice: walking is better than waiting for the shuttle bus, especially if your incoming flight is on time. However, when you reach baggage claim (or even better unless it's raining, exit from the terminal at the baggage claim level), turn right and keep walkingwith the terminal buildings on your right side and the roadway on your left.

If your incoming flight is late, I would suggest heading out to the bus area and flagging down the first car rental or hotel shuttle you see. They all stop at Term E after Term C, and it's usually not a problem to have them give you a lift.

The likelihood of your SFO-BOS flight being on time is fairly high, and because you're (presumably) in F on that flight you'll be among the first off the plane in BOS.

Unfortunately, LH's 2nd flight to FRA (dep about 9.30pm) doesn't begin until May 1st. So, if you do miss the 6.45 to MUC, you are probably out of luck.