Wiktionary
vb. (context idiomatic English) To disorganize or spoil something, especially an established arrangement or plan.
Usage examples of "upset the applecart".
These damned Engelists are trying to upset the applecart and take over.
There's been something lying quiet down here ever since pa disappeared, and we've upset the applecart.
She was right up on top of the load after a hard climb, and she was going to do nothing to upset the applecart.
Francesco Berger took over, and was so genial, so anxious to put Schnak at her ease, suggested so often that he approved of what had been done, that he almost upset the applecart.
But Data would have to be judicious he didn't want to upset the applecart of time, and he would be very, very careful as to what he did and how he did it.
That'll bring all of them down on anyone who tries to upset the applecart.
Sooner or later, after all, there would be a female President, and that would really upset the applecart, a fact well known but studiously ignored to this point in American history.