Set the proxy permission in yum.conf file in your system, please note that you must be on root privileges to update. Just follow the below steps,
local@host# su -
Normally yum.conf file is available under the /etc/ directory. Before editing the conf file you should be know the server IP Address, proxy Port number, proxy login credential.
Before editing a yum.conf file to take a backup using cp command.
Assume that in my local server IP Address is 192.168.0.1 and Port : 3128,
Have to add the line in configure file
Once it will be done save a file with :wq! and run the below two commands,
Eliminate any cached packages or old headers, if you execute the clean all will force yum to download all the headers and install at next times. Finally you can run update command.