How to install new updates

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I want to know how to install new updates ?
There are many files that is usually updated
modules-xxxxxxx.tar.gz
zImage
rootfs.img
system.ext2
froyo.user.conf

my question is .. it it enough to just replace files to memory card and when pre-installed Andriod starts it will use the new file
or I have to format the memory card and paste the files before install Andriod ?
Also it would be nice if someone can share any tutorial about these files and the role of each one ?

Updates that should not require formatting SD card (or other changes):
-> zImage and modules-xxxxxx.tar.gz - linux kernel image and the corresponding modules, they need to be updated both at the same time
-> rootfs.img - root file system image
-> initrd.gz - initial ram disk
-> XXXX.user.conf - additional boot options (swap/compcache/other stuff)

You don't have to but I would say it is better to remove the data.img file (data partition image) prior to updating system.ext2 (Android system). Android will create a new one in this case but all your personal data and settings will be lost.

You should be able to update pretty much everything w/o problems but in some cases something may go wrong. Always backup your data.img before updates - just to be safe.

Formatting SD card is required only when the file system is broken (one of the symptoms is Android's inability to read anything from the SD card).