So here we are, let's Go, it is simple.
How you start on this mission is entirely up to you. It may be more comfortable to start within a small area of influence, like in your home, in your circle of friends, in your street. You may already have some kind of area of influence and will be comfortable going bigger, like in your place of work, in your community, in your school, in your place of worship, or eventually in your town and your country, etc. There are no limits.
You may be a natural, or it may be difficult for you. Just be brave and confident and you will find your comfort zone.
We exist in societies
In order for us to co-exist with others in a society, there must be rules. It is best to accept the rules that have been developed over centuries and adhere to them. Accept the fact that they were made in the general interest of everyone's well-being. Yes, things change and rules must be changed, and will be changed, but in general, the rules are for everone's benefit.
Be nice
We all want to be treated "nicely". Treat people as you would like to be treated. Practice mutual respect. It is simple. Do not talk down, or up to anyone. Greet and treat as you would like to be greeted and treated. We are all equal.
Avoid conflict
There is so much conflict in our world. Is it unavoidable? In my opinion, there are two major contributors to conflict situations. When we expect others to think like we do. When we judge others. Perhaps judgement stems from what we think, not what we know as fact. We are all different, each one of us is unique. If others do not think like us, accept it. I believe most conflicts arise from others not doing things how we think they should be done. Leave them alone.
In a nutshell
So that is it. How do you change the world. Become a Model Citizen, by your Behaviour. Obey all the rules, practice mutal respect, do not be judgemental and avoid conflict. This may be more difficult than you think. When others notice your behaviour it will be a positive influence in the longer term. It will work. Do not give up. If you are really brave, get a T-Shirt printed with "Model Citizen Movement" on it. If you wear it in public, you will have to explain what it is and live up to it. Good luck, be brave.