Anything you do via
apt should (precluding errors, of course, or misuse, such as killing an operation halfway through) leave things in a consistent state. That's one of the main purposes of a package manager.
the first time I tried it failed
While this doesn't seem to have critical bearing on your question since apparently everything now works, beware that just saying "it failed" is rarely enough information for anyone else to provide you with a useful amount of accurate information in return.
the second time it seems to all going pretty smooth
I'm presuming by that you meant the upgrade. While you can
install, (or just
--reinstall) something to the same version (if you are aware or suspect something has been screwed up), an upgrade means replacement with a newer version; an unqualified system wide upgrade will replace all installed software if there is a newer version available. Again, part of the purpose of the package manager is to automate this as necessary, so no, you don't have to uninstall anything first or worry about it afterward.
apt-get update; apt-get upgrade (as you did) once a week or so is a good idea.