Take a Code backup as well as DB backup before upgrade. Use SSH to upgrade. Follow below steps to upgrade..
Open command terminal.
Go to your Magento directory by cd command like cd /var/www/yourmagento/ & run below commands step by step…
chmod -R 777 lib/PEAR
chmod 550 pear
perl -pi -e 's/rn/n/g' pear
./pear upgrade -f magento-core/Mage_All_Latest-stable
chmod 550 ./mage
./mage mage-setup . (* notice the dot after mage-setup command)
./mage upgrade-all –force
Open config.xml and change the statement SET NAMES utf8; SET FOREIGN_KEY_CHECKS=0; SET UNIQUE_CHECKS=0;
Empty your cache,sessions
Visit your site. It will take some time to loading depends on your data. Normally for 2 GB it will take 2 hours.