In order to reinstall Minecraft, follow these instructions based on your OS, keep in mind that this will not delete your current saves, or worlds.
To reinstall Minecraft on Windows, follow these instructions.
To reinstall Minecraft on Mac, first locate a program called "Terminal", Which can be found by opening Finder and browsing to Applications -> Utilities -> Terminal
mcd=\~/Library/Application\ Support/minecraft && mv $mcd\/saves ~/temp/saves && rm -fr $mcd\/* && mv ~/temp/saves/ $mcd
To reinstall Minecraft on Linux, follow these instructions
mcd=\~/.minecraft && mv $mcd\/saves ~/temp/saves && rm -fr $mcd\/* && mv ~/temp/saves/ $mcd