Tutorial: Installing Windows Phone 7.8 on your phone
Now if you are eager to watch the new OS Windows Phone 7.8 from Microsoft You can install Windows Phone 7.8 on your Lumia 510, 710, 800, 900 by downloading the firmware from navifirm.
NOTE: Technology Induced will not be responsible for any loss of data or device malfunction which can occur from this guide. Please proceed at your own risk. Please backup your data via sky drive or Zune before proceeding. Issues that can occur on your from following this guide will not be covered by your Nokia Warranty. Please know that you may void your device warranty if you proceed.
1) Download Navifirm from HERE
2) Extract the files and select the Correct device and the RM version
3) Now select the correct Model or you can search by product code which is written on the back of your device when you have remove the battery cover (i.e 059R0G3)
4) It will show a list of Files select and download all the files from it.
5) Now Install Nokia care suite from here
6) Go to > Program files > Nokia> Nokia Care suite > Drivers and install the WinUSB Drivers
7) Now go to C:\ProgramData\Nokia\Packages (Unhide if the folder is hidden) and make a new folder named "Products"
8) Make again a new Folder named with your product's RM (i.e. RM-889)
9) Copy all firmware files to the folder created
10) Open Nokia Product support tool
11) Enable the work offline mode in the file menu
12) Connect your Lumia device to your computer
13) Go to Product Support Tool and select [b]Programming > Refurbish.[/b]
Now the New the Firmware is installed on your Lumia device[b][b] [/b]