keep in mind he hasn't tried to tangle with the active military high command yet
the President can as C-in-C fire (I believe) any general officer from an appointed position i.e. the Joint Chiefs of staff
but it would surely raise a massive uproar from a variety of quarters, so whether he could get away with it is another matter
Matt Milley - the current CJCS - has been very explicit, both over the summer after Trump's threats to use the military on protestors, and a couple weeks ago before Congress, that the military isn't going to get involved in domestic politics, including the election