There are many parts of ACIM which SEEM to suggest you should not. Such as:
"Seek not to change the world, but seek to change your mind about the world."
Also Jesus tells us we need to correct the real cause of suffering in our mind, in our perception, in order to truly heal it.
Some say that this might possibly lead to a "side effect" of the body being healed, or it might not. Which is wishy-washy bullshit.
Let's break it down.
If you believe this world is reality, and as a world it sucks big time, and you hate it, and its insane, and you think it's "really insane" as in, it actually IS, then YOU have a problem in your mind because you're not seeing correctly. The world is an ILLUSION!
If the world is real to you, then you've mistaken illusions for the truth. You've given reality to illusions. And now when you see an illusion of a "bad world", you're going to react to it with despair, depression and hopelessness, only because you think it is REAL and not because of its form.
If you then seek to change the world to "look better" AS a real world, which is what happens when most people try to change the world, you're actually going to KEEP the belief you have that this world is reality. You will move the furniture around, maybe to some configuration you prefer temporarily, with tradeoffs, but it's not necessarily going to give you real lasting happiness. It will give you a temporary illusion of happiness which is dependent on the world, and it will change on you.
There is a different way to change the world. Perhaps we should say, to TRANSLATE or TRANSFORM the world.
When you withdraw your belief that this illusory world is reality, and you stop taking it seriously, and stop reacting to errors, sins and sickness as though they're real, you will be much happier. You will know there is not REALLY any reason to be upset, that the world is not REALLY bad, and it's not REALLY anything. This will alleviate all possible reasons for upset.
From that vantage point you will see the "forgiven world", which you will be OVERLOOKING, to Christ's face which is beyond form.
You will also be miraculous, and with the power of God, you WILL have access to be able to "change the world" in any way that is appropriate. Albeit that perhaps it would need to be a) inspired by love, b) in alignment with Jesus's plan of atonement/awakening, and c) for the good of all.
Just as Jesus performs intercessions/interventions by way of MIRACLES, so can you, and you may well bring about "changes" in the world as a result. You may well seem to make the world a happier place, not because its FORM is better, but because there is healing taking place. People are waking up to the truth, miracles are educating about God's reality, and belief is shifting away from illusions. That's a helpful way to change the world.
Thus it's not then so much about to staying IN the world and fixing it up and making it all pretty and real, but more to do with you converting it to an illusory yet more tolerable "happy dream", which smooths the way to awakening and eases suffering and gently helps you and others to return to God. This "use of the world" for awakening is a better purpose than to try to "use the world" to produce a "real world" that is considered better than the old one.
There's nothing wrong with you changing the illusion. Jesus did it many times. Every miracle does it. Miracles rearrange space and time. Jesus tells us its a device for the manipulation of time (and space)! Why not ease the nightmare and lift some burdens lovingly so that people can see EXAMPLES and DEMONSTRATIONS of right-minded thinking, the truth of God's kingdom, the POTENTIAL of awakening, and the FREEDOM that is everyone's right!
The purpose then isn't to change the world for the world's sake, but to change the world to help people wake up from the world and return to reality.