Wouldn't make a difference. The only part that could change my actions would be what hapoens obce it ends. But simulation or no I'm stuck here for however many years and have to make the best of it.
It wouldn't change anything because this is the reality we have. I wouldn't suddenly become immoral because there are still consequences and even if some people or even everyone but me is not real, they will still react as though they have emotions. If I hurt my wife, she would still cry. If I did something bad enough, she might divorce me. Take half my stuff. Lose some access to my kids. Those are all real consequences even if the people are simulated.
Maybe I'd be less concerned for people not immediately around me because any good simulation wouldn't actually sim everything out of range, so there wouldn't e.g. be Palestinians starving to death if no one "real" was there.
I suppose it would re-open the question of existence beyond death, but I don't think I would ultimately come to a different conclusion—there is no reason to think this experience would continue past my own death, and if I were memory-wiped and re-inserted it wouldn't really be the same me anyway, right?
If it were matrix-style there is a reality out there to experience if I can leave the simulation, I'd have some hard thinking to do about whether that would be likely to be a better existence than the one I'm having here.
Also I'm not sure if I'd me more or less scared of death. If this is a simulation, to escape reality, I can't imagine how terrible reality would really be.
I'd give the sky the finger and then spend too much time wondering what perspective we're even being viewed from.
I'd also probably start a diary, except every entry would be: "Fuck you people for creating a world like this. Do you enjoy the suffering of countless people? Turns out God really is a prick."
I think that two items would be pretty prominent for humanity:
First, if we know not just that it is a simulation but that this brings some knowledge of how the simulation's physics work that differs from our previous non-simulation understanding, how to make use of those properties.
Second, figuring out how to "break out", how to influence the world in which the simulation is being run.
If I can't determine what lies beyond the veil, then nothing much changes, does it?
If I'm presented with a beautiful utopia, or just another world that's not a hellscape that lies beyond our simulation, and then am thrust back into the simulation, then I guess I'd be less afraid of death. If I'm presented with a hellscape, then I guess I'm more scared of death.
However, if I could somehow be convinced that this is all just a simulation, but can't see what lies beyond in what is actual reality...then that's just like...knowing there's an afterlife, but not knowing if it's better or worse than this plane of existence, so it's practically worthless information.