First Step must have Zune ... download from here ..
And then connect your Nokia Lumia to PC with Zune application ... wait until show update proses ...
NOTE :
If you check your Nokia Lumia but still have Windows Phone 7.5 just try again your update with Zune until you get windows Phone 7.8 ...
Next step after you get Windows Phone 7.8 , factory reset your Lumia from Setting - About - Reset your Phone ....
Get Bluetooth Share at Nokia Collection ...
... source : ... http://ruk.ca/content/how-i-updated-my-nokia-lumia-800-windows-phone-78
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