You shouldn't uninstall yum (because it doesn't do any damage unless you run it, and not even then every day), but it's your own decicion
Well, the lesson obviously is that you should check what you are upgrading before going on. Especially if it's an "important" system that you can't reinstall just like that.
You can't "undo" the changes yum made, because "installing" or "upgrading" packages means that new files are copied to the system (the "package managing" part means that those changes are tracked, so that removing a package means removing the files that were copied to the filesystem when the package was installed), and in case of upgrade, any existing files are overwritten. If you don't have the old versions available (yum reposities only hold one version of a package, the newest one on that reposity, so your chances of finding a reposity with older packages are pretty small), you can't "downgrade" by installing the old packages, so your only options are to manually fix the problems (find the exact problems first) or to try to reinstall the packages and hope that it helps (probably not, if the upgrade itself worked out, but the packages just didn't work). A third option is to wait until new versions of the packages come to the reposity and then re-run yum upgrade to see if a new upgrade helps.
Typically upgrading trough yum should not create problems, but it is possible. You can try to reinstall the packages one by one if you think it helps, but I'd make sure your backups are in place, because if you can't figure anything else out, a reinstallation might be the quickest help
Though if the problems are somewhat minor, and you have time for them, try to figure out what exactly is wrong with the apps and fix that. A kernel upgrade means that you need to reinstall any drivers that were added after the last kernel installation, like graphics card drivers, so you could just as well start from that..