Hi Ragunath, only these platforms support 15.7: BlackDiamond X8 and BlackDiamond 8800 series switches, Summit X770, X670, X670-G2, X480, X460, X460-G2, X440, and X430 series switches. E4G-200 and E4G-400 cell site routers. If you have one of them, I doubt there is a way of making a hitless upgrade from any 12.X release straight to 15.7. You can export your configuration to a TFTP server and import it back to your installation