Using nightly build feed
Add https://www.myget.org/F/avalonia-ci/api/v2 to your package sources:
Update your package using Avalonia feed:
If you experience some problems try to switch NuGet to v2 feed - http://www.nuget.org/api/v2/:
If you still can't update to nightly version try to uninstall Avalonia package and install it again.