Electoral College
Amendments proposed by 2/3 Congress
Ratified by 3/4 state Legislatures
"Conventions" is a possible fly in the ointment, though.
It took me a bit to see it, but now I want to go back to what I first saw.
I think anons implication was that Trump was supposed to be taken into custody by military people there. At one point there was movement as if it was going to happen, then they turned around, hence Obama's concerned face.