Updated OS install guide

Admin

(Optional) Uninstall any previous OS from PC.
Download OS to PC. Use Internet Explorer, not Firefox or Chrome. (For leaked versions, browser is irrelevant.)
*If 100% certain you've previously used FF or Chrome on same PC to DL an OS successfully, feel free to use FF or Chrome again.
Following successful DL, run OS file to install OS on PC.
On BB, go to BBM Options, scroll down to Backup section.
Ensure Remote Backup is enabled AND also conduct a Local Backup to media
card.
Go through any 3rd party apps with option to save settings to
media card. Conduct Backup to media card within each of these apps.
(Examples: Bellshare apps like BerryBuzz & BerryWeather;
QuickLaunch; Shao's apps like Wallpaper Changer & SixTools; etc).
Connect BB to PC. Open DM and backup device. Close DM.
(Optional) Use desktop app like BBSAK or BBMCP to backup 3rd party apps.
On PC, delete Vendor file located here:
•64-bit: Computer/C:/Program Files (x86)/Common Files/Research In Motion/AppLoader
•32-bit: Computer/C:/Program Files/Common Files/Research In Motion/AppLoader
(Optional/Recommended) Run BB Boss v2.2 or BBH Tool (formerly Shrink-A-OS).
In same AppLoader file where you deleted Vendor, double-click Loader to load OS to BB. Follow onscreen instructions.
After loading OS, perform several physical battery pulls over
first 1-2 days for OS to "settle in." Perform first batt pull very soon
after OS is loaded.