Amr,
When purchasing your Cisco switches, I hope you have a service contract for them. If you do, you should be able to create a user account on www.cisco.com which should provide you access to the the software download site.
If you do not have a service contract, You best solution may be going back to the vendor that sold you the switches. If you purchased directly from Cisco, Contact your local Cisco Account Representative and they should be able to help.
Cheers
T.
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/6-x/upgrade/guide/b_Cisco_Nexus_9000_Series_NX-OS_Software_Upgrade_and_Downgrade_Guide_Release_6x/b_Cisco_Nexus_9000_Series_NX-OS_Software_Upgrade_and_Downgrade_Guide_Release_6x_chapter_010.html
https://supportforums.cisco.com/document/12268066/nexus-9000-standalone-nxos-aci-conversion-and-back
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/kb/b_KB_Converting_N9KSwitch_NXOSStandaloneMode_to_ACIMode.html