If you own a Nexus then chances are you are pretty familiar with flashing a stock image to your device. Last week Google released an all new 5.0 Lollipop Preview build. There are several changes, especially in the Material theme. Installing this is actually pretty simple. You will need to have an abd/fastboot environment setup on your PC or mac. Normally this means having the adb.exe and fastboot.exe files stored in a folder. If you don't already have that set up you can find some pretty simple instructions here.
Once you have the adb and fastboot environment setup you can head to the link below to grab the image file. This will be in the form of a TGZ. You will need to extract the file. then copy the 5 files within the folder and then paste those into your folder containing the adb and fastboot files. Once you have copied and pasted the files simply double click the "flash-all.bat" and the install script will begin. Your phone should be in bootloader mode (from power off, press and hold volumedown+power), and should be connected to the pc.
Download Android 5.0 Lollipop