If Biden loses fairly, should he concede, or should he use the military to remain in power?

[Thank you for being a friend...]

lol I don't think any Democrats are concerned with Biden trying for a coup.  The man is very much a traditional pol of the basic establishment.

And that's a good thing - if you lose as an incumbent, you walk out the door and let the other person in.  You don't conspire and scheme to stay in.  That's how things are done in other countries, but not ours.  

Look at Carter and Reagan...Bush the father and Clinton...Bush the son and Obama...Obama to Trump... these preceding elections were bitterly fought, but the men themselves had to put it aside to allow for a transition.

The office of the Presidency is bigger than one man - and George Washington being the first president knew that and likewise he knew what had to be done to start the country off the right way.  That was probably harder to do than one would think - power can be very corrupting.