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 -
password :

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 and Port : 3128,

local@host# cp /etc/yum.conf  /etc/yum.conf_backup

To be edit the configure file with text editor like VIM or VI tool

local@host# vim /etc/yum.conf  ( or )  vi /etc/yum.conf

Have to add the line in configure file

Once it will be done save a file with :wq! and run the below two commands,
local@host# yum clean all

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.
local@host# yum update