How to get KDE 4.8 ??

how can i upgrade my kde version to 4.8 ???

(1) I’ll shift this to a help forum

(2) Attach the relevant factory repo and run the command as root: “zypper dup”

thank you!
what is the relevent factory repo?
can you give the link of it?

For KDE 4.8 you can use the Upstream repos: KDE repositories - openSUSE

Add and enable the Core and the Extra repo, and then use the repo switch in YaST to switch system packages to these repos. If necessary, do a repo switch back to Packman afterwards. How the repo switch works can be seen here:
SDB:Vendor change update - openSUSE

There is actually a thread on KDE 4.8:
Should address some difficulties which might occur during and after installation, for example minor dependency issues.

now is it stable to install 4.8 ??

That’s a question that you have to think on. Have a read of this thread and make a decision based on the experiences there:
Really, it all boils down to (a) your own personal risk tolerance and (b) your proficiency with Linux.
I installed it in openSUSE Tumbleweed and had three dependency issues to solve. It works nicely for me in Tumbleweed (so far). But Tumbleweed is not 11.2.