Huawei has had a complicated relationship with the Ascend Mate2 and its software update. It originally announced that the device would stay on Android 4.3 last year, but then reversed itself and said the Mate2 would jump straight to Lollipop. It's taken a long time, but there's finally a public version of this update, but it's still technically a "preview."

2015-07-01 14_02_42-Mate2 Android L Update Now Available - Announcements - Support - Huawei Communit

The update is based on Android 5.1 and comes with a new version of Huawei's EMUI layer. There's no OTA, so you're going to have to flash the new ROM manually using an SD card. Here are the instructions provided by Huawei.

  1. A micro SD card of 4GB or more is required.
  2. Format the micro SD card (This is optional).
  3. Download then extract the B309.zip file.
  4. Copy the entire "dload" folder to the root directory of the micro SD card.
  5. Make sure your handset is powered off. Insert the micro SD card into the handset.
  6. Press and hold the VOLUME UP and VOLUME DOWN keys simultaneously, and then press the POWER button. Release power when you see the device turn on.
  7. The phone should boot and begin flashing. When the progress bar stops, the phone will restart.
  8. You may enter the Emergency Data dialog at the first boot. Choose the Factory data reset and click the Yes button. When the reset is done, reboot your phone.

It's not terribly complicated, but it seems like there are a number of ways things can go wrong. You need to be running the B126 or B148 versions of the official software to install this update, and future versions may or may not require a device reset. If things go wrong, Huawei has provided a Jelly Bean downgrade package as well.