I am right now running Unity 4.2.2
I just saw that Unity 4.3 has been launched.
How do i upgrade to Unity 4.3?
Just install Unity 4.3. The old installation will automatically be removed then. When you want to keep the old Unity 4.22 installation, then you have to install Unity 4.3 into another folder. They coexists then.
And make sure to make a backup of your project before you install Unity 4.3 in case you have to go back to 4.2. Which you probably have to. There are some showstopper bugs in Unity 4.3 at the moment. So i would wait with the upgrade a bit longer until 4.3.1 arrives.