There is a way to force the Mango update early through Zune software.
(Please leave below a comment if it works (or not) for your device. Thx in advance!)

Here it is:

Before we begin, keep in mind that a total of three updates must be installed on the path to Mango: 7392, 7403, and 7720, which is Mango itself. The process will take a while. Some people may already have 7392, in which case things will move a bit faster.

Steps to force a Mango update on phones for which Mango has started rolling out:

  1. Connect your Windows Phone to a PC and launch the Zune software
  2. Click on Phone/Settings/Update. Allow the software to tell you whether an update is available. If so, skip this guide. If not continue to the next step.
  3. Click the update button again, and this time immediately disconnect your computer from the internet (some people have better luck waiting 1-2 seconds). Do this by pulling the Ethernet cable, disabling Wi-Fi, etc.
  4. Wait 30 seconds or so and you should receive a notification that an update is available. This is 7392, the Mango pre-update.
  5. Now reconnect to the internet and continue with the update process through the Zune software.
  6. After 7392 is fully installed, you may receive a notification that another update is available. If so, install Mango now. If not, proceed to the next step.
  7. If the second update isn’t popping up on its own, repeat steps 3-5, causing the 7403 update to appear.
  8. Install 7403, brave reader.
  9. One more time! Check for the last update, Mango. If you see it, go for it. If not, head to the next step.
  10. If the Mango update isn’t popping up on its own, repeat steps 3-5, causing the 7720 update to appear.
  11. Using the Zune software, install that Mango like it’s hot.
  12. This last step is very important! Celebrate your phone’s new found Mango status by purchasing a mango smoothie at your nearest Smoothie King, Jamba Juice, or Baskin Robbins.
Force Zune Mango Update

Force Zune Mango Update

Via: www.wpcentral.com