The Windows 10 Technical Preview was released this Thursday, but only for select phones. Now thanks to an XDA-Developers hack you can install it on just about any Lumia that you are not using.
That last bit is important since this can brick your device. People have reported success with various phones, including the Lumia 720, which has just 512MB RAM. The update is officially available for 512MB phones, though there's a possibility they won't get all features in the final build.
Anyway, the update has worked on a Lumia 930 and a Lumia 1320 bought from different carriers, though some are reporting issues on Lumia 920 and 520.
You can read the step-by-step guide for the update in this thread, plus reports on how others fared and some troubleshooting. The process is fairl involved and not for the faint of heart and once again it could brick your device.