hi all, i am using fedora 11 now. i dont know what happens when use "yum upgrade". is it to upgrade fedora version 11 to 12 or more? please reply me.

Try to google "fedora yum upgrade".
It's pretty simple.

the RECOMMENDED way to "upgrade" fedora is to do a clean install from a new burned fedora 13 dvd
the" upgrade" and the NEW ( and recommended over upgrade) "preupgrade" mostly work .
Some have not had any MAJOR problems , just the regular normal problems when upgrade is used .

burn fedora 13 dvd and reformat the disk to the new ext4 format that fedora 13 uses

PLEASE READ the fedora docs page -- it is there to answer questions
http://docs.fedoraproject.org/en-US/index.html

When performing upgrades I've always preferred to keep a level of safety in mind. While a yum upgrade approach is possible, and I'm sure people have done it without incident, it just seems too much of a risk to me. I use the same approach as John VV by performing a fresh install. I also take it one step further by having the current and previous release available in different partitions of my hard drive. This provides a reference point and makes reconfiguring the new system much easier. It also allows for a fallback position in case something in the new release gives me a problem. One other thing I do is keep notes of the changes I make to a system. With release dates 6 months apart, the notes help me to upgrade to the latest version in a relatively short period of time.

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Bill