Then tell them how the candidates' stands will affect them. Or how the candidates' histories are relevant.
For instance a while back I was talking to a young man who is starting his own business. He was undecided but trending towards Trump. So I talked to him about Trump's horrible history of stiffing small businesses and how Trump hates to pay the agreed upon price for services. And I brought up the thousands of lawsuits against Trump for that kind of thing. That really disgusted the man!
Since he was not that impressed by Hillary Clinton, I didn't try to change his thinking other than to tell him that she's been the target of Republican hatred and investigations since the 1990s and nothing has ever been found that she has done wrong. He said something about Benghazi and I told him there have been 9 different investigations and not one charge resulted from them.
I don't know for certain that I changed him from a trending Trump voter but I did get him to start thinking.
If not, find them on YouTube and watch them.
Also, having them watch an entire Trump rally speech is effective.
Then ask, does that seem like the kind of person who should be president, even if you agree with some of his positions?
The second thing would be to call them a liar about being undecided.
"If you choose not to decide, you still have made a choice."