I am an extreme beginner in Blueprints, hence sorry if my questions is trivial. How to change only the z-component of an actor’s location ?
Like this:
You can right click and split pins, too 
. Or like so:
Kind of depends on what change means. Because you can also:
If the actor is in the world, you can mouse over transforms components and drag the values around with the mouse:
Or punch value in. Or you can use the Gizmo:
my questions is trivial
I’d say it’s more on the vague side of things, as I’m sure there’s another half a dozen ways to do what you’ve described… 
Thanks for your detailed answer. However in all example the target location needs to be fed with x, y and z. In case I just want to change z dimension of actor location, do I have to use a get location, change one part and feed set location ? I’m referring to Blueprint mainly. Thanks