Android SDK Manager

To update the android SDK and the Tool in visual studio you have to use the Android SDK Manger to access this manager go to tools->Android-> Android SDK Manger

once this is selected this will launch the android SDKs and Tools application, here you can install various platforms their code names,  version numbers, and most importantly their API levels.

now if we switch over to the tools tab we can see the various "tools" that we have install and that have pending updates.

This where we can see our sdk version, keep in mind that the sdk is backwords compatible and thus we should try and keep it up to date.

we can also leverage platform/version specific features by inspecting what platform and version our device is.

var version = Android.OS.Build.VERSION.SdkInt;

if (version >= Android.OS.BuildVersionCodes.Kitkat)
{
    // do things supported by kitkatversion
}