Your Actor is the object that has the material you want to change textures on. You can cast your Actor to a StaticMeshActor as shown in image 2 (this is done to get the StaticMeshComponent, which is the target of SetMaterial). Just use a variable to store your Actor (in image 2 it is called MyActor).
Step 1 and 2 must be done first. During runtime, when you are ready to change the texture on your material, you simply use Step 3 - this could be at the press of a button. I’m sure you can find a guide on how to set up a blueprint button somewhere in the Unreal 4 examples or on the forums. Right now I’m just using the event called when the player presses the fire button (InputFire event).