How do I change dificulty of my monsters while game is running

I am trying to add more health to my zombies which are spawned as a wave after a button click

ibb.co/zXN5KFg

The variable changes as it is specified in the actions but the spawned zombies still have the initial health value

So my question is..can I update the IV mid-game so the change applies to the newly spawned zombies or do I create new entity with desired properties and change the spawning events?

Your mistake is that you are adding 1 to existing zombie instances, before new zombies are spawned.

You need to add a global variable ZombieBaseHealth and add 1 to it with every new wave. In the event where zombies are spawned add an action "Zombie Set Health to ZombieBaseHealth".