If you are planning to keep untethered jailbreak on iOS 4.2.8, stay away from this update as far as possible. Make sure you install PDF Patcher 2 from Cydia which basically closes this PDF hole after untethered jailbreak, giving you the same effect of iOS 4.2.9 on 4.2.8 with fill untethered jailbreak.
For those of you who want to update, simply connect your iPhone to your PC or Mac and start iTunes. When prompted for update, click on “Download and Install” (Mac OS X) or “Download and Update” (Windows) to get on to the new 4.2.9 firmware.
Download iOS 4.2.9 for CDMA Verizon iPhone 4 (Direct link official from Apple)