Although we should not overstate this, the Renaissance can be regarded as a revolution because it fundamentally changed modes of thought in Western Europe.
The Renaissance can be seen as a revolution because it started a movement away from a vision of the world that was centered around God and the Church. During the Renaissance, people started to think more about natural processes and scientific laws. They started to think more about the human being as the most important thing in the world.
It is not right to say that all of this changed completely and quickly. However, the humanism of the Renaissance did start these changes in motion. Before the Renaissance, the Church and God were the center of most people's lives and were the source of authority on all sorts of matters. By the time the Renaissance was over, people were much more likely to think of humans and human society before God and were more likely to look to science for answers about the material world.
This was a huge, if gradual change. Since the Renaissance set it in motion, the Renaissance can be regarded as a revolution