1. Download USB drivers, RSDLite (32bit or 64bit) (click here)
2. Choose the right .SBF file ()
3. Connect your Milestone to PC and Turn it off
4.Boot your phone with boot loader by pressing D-Pad Up Key and Power Button (At the same time)
5. Launch RSDLite and you shall see Milestone on the list
6. select your .SBF file and start the process.
The phone will reboot itself once everything is done. Congratulations! your Milestone is back to stock version.
If facing any kind of problems, kindly log on to XDA official thread!