Okay So Here we go :3 , this tutorial is made for lumia 620 but also should work on other x2x devices

IM NOT RESPONSIBLE FOR ANY DAMAGE TO YOUR DEVICE / SD CARDS !!!!!!!

Step 0 Upgrade to Windows 8.1 mobile first , if ur already on w8.1m dont do that
First find&download ffu file from here

U should find category for your model Like RM-XXX

After that u should search NOT FOR YOURS exact phone as it will have windows 8.0 and not 8.1 , u may check as sometimes its provided by producent

the version it needs to be somthing like this : 3058.50000.1425.XXXX , if its like 3056 and not 3058 its win 8.0 and update wont work

Please Flash It now with WPinternals

after that please upgrade To newwest w8.1

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Step 1 Upgrade to Windows 8.1 Mobile GDR2

im lazy so here is

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Step 2 Upgrade to Windows 10 Mobile (early builds)

Now Using Universal Upgrader Push Windows 10 mobile (the newwer one that shows up)

This time Windows 8.1 will not display error and will install windows 10 mobile successfully

Universal Upgrader DONE !

U may now push newwer windows 10 mobile builds upgrades