Upgrading to a specific version of android

Updates are device / carrier specific - there isn't an update that works on every device or any device, si there's no question of an universal repository

If there is an update available for your device, it can be activated by clicking the System updates in the about phone /device portion of settings (usually, but you may find it elsewhere in your settings

If it says there is no update, then you can't (though in some cases OEM hosts on their site an update to be manually downloaded and applied, but it's more of an exception)

If you want to upgrade at any cost, you will have to root your device and flash a custom ROM