Earlier quoted context omitted.
If every single person on Earth did 1 - 4 it still wouldn't be enough. Change has to come from industrial polluters to have a real impact, and the only way to change industry is to stop driving growth through consumption. So... 5) Stop buying stuff .
I think if every single person on Earth went vegan it would have a significant impact. It would force the meat industry to change.
If you still want meat in your diet, start buying less beef and more eggs or chicken. That'll be a good start. https://timeforchange.org/eat-less-meat-co2-emission-of-food