Error while installing SDK 21

tell me how to solve it

Use the microsoft process explorer to determine which applications are locking the files and get them to release the locks. Press Ctrl-F after opening process explorer to search for open file handles.

Thanks
by your suggested process cmd is closed but when i try to close java.exe all the eclipse components gets closed
so it didn't get updated

Oh my. That is troubling indeed. Try to change the directory that ADT installs to. A fresh directory will ideally have no locks on it. If the installation fails again, it might be a bug.

You might try getting the SDK parts from the Android SDK Manager rather than eclipse. The SDK Manager should be in your start menu. In the three systems I build Android on (2 Windows, 1 Linux) I did it directly through the Android tools rather than Eclipse.

How to change the directory of sdk?

The plugin asks you for the installation directory in one of the screens. Change it and give it a try or take Steve up on his suggestion.

@steve: at my start menu there is only uninstall android sdk option is available