I`ve installed Xamarin (Xamarin studio, Xamarin.Android) But there are some problems with building even the simpliest project(just create new project->Build->Get an error).<
easiest way of doing is as described here
Download links Links for the recent individual installer packages can be found on https://store.xamarin.com/account/my/subscription/downloads.
In most cases, uninstalling is not even neccessary. just download the version you want to install and it will handle uninstall-install together.