You do not need neither root nor an unlocked bootloader to use flashtool.
Get flashtool from here (make sure to use 0.2.9.1 beta):
Get newest FW here (4.0.1.A.0.283):
When flashtool is downloaded just extract it to your desktop on your pc. Then extract your downloaded FW and put it in the folder named 'firmware' that came with the flashtool download. Then install ggsetup.exe that also comes with the flashtool download (it's a driver package).
Then hit the x10FlashTool.exe. A window will open. Press flash. Another window will open. Choose your firmware. Here you can choose to tick the 'wipe data' box. If you do you'll loose al apps and data stored on your phone. If you don't the apps and data will stay. In the latter case you might run into issues as FC'ing of apps and such, but not sure as people has done this without issues. It's recomended to wipe data but not a must, it's up to you. I would do it but then again I'm also rooted and can restore easily after every flash.
Anyway either way you choose you then press ok and just follow instructions shown on screen.
Remember after flashing and booting (wich can take some time) to enable debugging and unknown sources.
. Nokia N9 (sold). Xperia Play (refund). HTC Desire HD (sold). Nokia N900 (sold).
Galaxy Note 10.1 (will get it, late october
What you give is what you receive