The guy already blames everyone except for himself for everything bad that happens to him, so why would he accept defeat graciously? He'll either play it off like it's "no big deal" all while insulting the "rigged election" and voters, or he'll go after everyone and everything. He's not going down without a fight.
Trump believes that he can win with what he is running with, and his supporters will carry the day in the end. Trump will accept his victory graciously, and that is all that matters. Then again, his main competitor is a career criminal, so no matter who he has on staff, he is infinitely more desirable as a leader.
Trump has never done anything graciously in his life. He will continue to be the loud-mouth blowhard all the way up until election day, and then he'll complain about the process being rigged, or the media, or whatever else he fixates on at the time. There will certainly be no McCain-style concession for Americans to come together under Hillary.
Trump is unlikely to accept defeat and go quietly. He is more likley to blame the media, the voting system, voters, his campaign team and Clinton's team. There is no way he will accept that the outrageous things he has said and his lack of credible policies might be to blame.
Trump isn't going to change how he acts when he loses by a landslide in November. He can't handle defeat, in fact his ego is so big he is convinced he can only lose if the other side cheats. That leopard is not going to suddenly change his spots, especially when he has the spotlight on him.
Donald Trump will not accept his defeat graciously. He is already talking about the "rigged" system, and how the election will be stolen from him. This comes about despite his offering no proof of any such conspiracy. In his world, losing the general election would equate him to being a loser, which he cannot accept.