I installed i3 with sudo apt-get install i3. But if I try to launch it via terminal, it says another window manager is running. In older versions of Ubuntu, there was a round button at the login screen in which we click to select WMs. But in 18.04, there is no such option. How do I launch i3 without making the mess like changing stuff in a file?
Either Log Out or Restart the computer (I recommend a Restart if you are the only user on the machine), and then during the log-in screen, right next to "Log In" button, you will find a small gear icon which will let you change your Window Manager. Keep in mind that this Gear icon will not be visible when you try to access it from the lock screen.
For those of you that are not taken to a login screen, you may have to Disable Automatic Login, it is enabled by default. You can do this by going into: Settings -> Details -> Users -> Turn Off Automatic Login