Use sudo or su or whatever to switch to root
If you want to install the remi repositories for PHP for example
wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
wget http://rpms.remirepo.net/enterprise/remi-release-6.rpm
rpm -Uvh remi-release-6.rpm epel-release-latest-6.noarch.rpm
Now in order to update use the remi as well as the default repos, first check what repos are installed, enable them and run update. Some examples below