There are many ways of doing this.
Easiest might be to create a colliding volume that covers the height you want and set it to overlap dynamic actors.
In the “event on overlap begin” event in the event node graph, you can look at the actor that overlaps, and set its position.
Another way of doing it is in Event Tick in the actor itself. Get the position, break out the vector struct variables, get the Z, add a “>” node with 500 as the value, and wire that into a “switch” node to do whatever you need the action to be.
Have you actually followed the various blueprint tutorials on youtube? The kind of question you ask is really quite simple, so it sounds like you would learn a lot from completing them: