Upgrade your Nokia Lumia to 7.8

If Zune is unable to upgrade your Windows 7.5 phone to 7.8, you will have to do so by forcing an update.

1. On the phone – turn off Wi-Fi and Data. You can do this manually for each one, but activating airplane mode does both at once
2. Connect the phone to PC, and in Zune. Click Settings --> phone --> update – and let it fail.
3. Now, go back to select update – but this time, after a few seconds – disable your internet (disconnect the wire, turn off Wi-Fi – which ever one applies to you). The timing is totally random — I would try disabling after 2 seconds, if it fails, enable internet, and try again – but this time wait 3 seconds, if it doesn’t work – repeat – but increase your wait time before disabling internet. It can be frustrating, but it will work with persistence.

** The trick for the update is counting the seconds it takes to get to the “no update” message and then going off that tab to another and back, and disconnecting 1 second before the "no update" appeared the first time. 

** For my Lumia 710, I had to install 3 updates before it finally made its way from 7.5 to 7.8. 

** Credits: MobileSyrup forums

Upgrading to 7.8 will give your phone a new lease on life. If you're still on the fence, check this video out.