Hi Kishor,
There are several resources explaining how to upgrade the Fortigate's firmware.
I leave you some examples:
Cookbook - Upgrading FortiOS
http://cookbook.fortinet.com/sysadmins-notebook/supported-upgrade-paths-fortios/
Cookbook - Upgrade from 5.2 to 5.4
http://cookbook.fortinet.com/updating-your-fortigates-firmware-54/
Cookbook - Upgrading the Fortigate's firmware
http://cookbook.fortinet.com/updating-fortigates-firmware/
Also, in the Fortigate's Online Help you can review the guides:
FortiOS 5.2
http://help.fortinet.com/fos50hlp/52data/Content/FortiOS/fortigate-system-administration-52/Firmware/Installing%20firmware%20from%20a%20system%20reboot%20using%20the%20CLI.htm
FortiOS 5.4
http://help.fortinet.com/fos50hlp/54/Content/FortiOS/fortigate-best-practices-54/Firmware/Performing_Firmware_Upgrade.htm
FortiOS 5.6
http://help.fortinet.com/fos50hlp/56/Content/FortiOS/fortigate-best-practices/Firmware/Performing_Firmware_Upgrade.htm
Regards
