there should be a thread in the 8800 forum that answers that question. otherwise you should get your updates from your carrier's support site. create an online account with ATT and you should get access to software updates.
i dunno about the xml file... but would suggest you don't try anything esoteric until you're quite familiar with the device.
welcome! good luck!
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8300 Curve: viigo, gmail, google maps, way too many emails/day
Yes, there is a thread Stickied at the top of the 8800 section with the latest OS for this model. You can use it, follow the direction as instructed to load to your device.
The purpose of the deleting the vendor.xml file is to all the OS from another carrier to load to your BB, which may not be branded to the carrier which released the OS. Does that make sense?