It seems as 3.4 currently isn't in the Debian repository.
1.) You can always try to install manually (or using Konstruct)KDE home
but how about waiting 'til it's in the repo's? I would advice you to wait since you're pretty new to Linux, upgrading KDE manually is not that easy.
2.) You can safely remove them if you don't want to keep just in case you reisnstall the program again.