Hi Alessandro,
You can download the internal build from your account -> Products & Subscriptions -> UI for Windows Universal -> Download installer and other resources -> find UI for Windows Universal and choose Latest internal builds.
However, we will release the official Q1 2015 SP1 this week, so my advice is to stick with the official version which is more stable and includes additional fixes and features.
Please, let me know should you have any other questions.
Regards,
Ivaylo Gergov
Telerik