New versions always secondary-install like that. It’s good to do , since it means if something breaks in the transition to 4.6 from 4.5.1 you can fall back to older versions.
When the time comes, just select “remove engine” under 4.5.1’s drop-down in the launcher (or something similar to that text, I can’t recall exactly) to delete all of the engine files for that version.
Again, I wouldn’t do it right away though. Since I started with UE4.2 every major new release has broken something in my project and I’ve had to go back to the old one for reference until I can nail down the cause and make the appropriate changes. Just part of the process with software like .