Permission denied

hi iam getting this error while uploading.please guide me what should i do now

Arduino: 1.6.8 (Windows 8.1), Board: "Arduino/Genuino Uno"

C:\Users\cns\Desktop\arduino\hardware\tools\avr/bin/avr-ar: unable to rename 'core\core.a'; reason: Permission denied

exit status 1 Error compiling for board Arduino/Genuino Uno.

This report would have more information with "Show verbose output during compilation" option enabled in File -> Preferences.

Use 1.6.5. It is the last version known to run without bug.