[CentOS] Centos update 6.8 to 7

Sat Jun 18 21:50:18 UTC 2016
Bachir Bachir <bachir_bac at yahoo.com>

 I downloaded Centos 7 and i want to update from local usb i created using dd if=cenots7 of=/dev/sdc i dont want to reinstall, i need just to update.What would be the best way to do so pleaseThanks muchBachir