Upgrade Bugzilla 4.0 to 4.2

With the latest release of Bugzilla, you get many features.
Such as…
HTML Bugmail
Improved Searching System
Accessibility Improvements
Auditing of All Changes Within Bugzilla
Displaying a Custom Field Value Based on Multiple Values of Another Field

Release Notes

Make sure you have all dependencies covered, such as Mysql, perl modules as mentioned in release notes.

You can refer Official Upgrade Doc Or below procedure.
Here, we assume you have 4.0 previously installed as following is tested on Bugzilla 4.0.x running on CentOS 6.2

Before you start with upgrade procedure.
Make a note of all changes you made in custom templates & delete it. As older templates will be replaced with newer version.

rm -rf /var/www/html/template/en/custom/*
 
Download Bugzilla 
cd /root/
wget http://ftp.mozilla.org/pub/mozilla.org/webtools/bugzilla-4.2.tar.gz

Backup Existing html folder (or bugzilla folder)

cd /var/www/
tar zcvf bugzilla-40.tar.gz html/

Take MySql Backup

cd /root/
mysqldump -u root --databases bugs > bugs_back.sql

Extract Bugzilla

cd /tmp/
tar zxvf /root/bugzilla-4.2.tar.gz

Copy Content of bugzilla-4.2 to html / bugzilla folder

cd /var/www/html/
cp /tmp/bugzilla-4.2/* ./ -rfva
cp /tmp/bugzilla-4.2/.bzr* ./ -rfva
cp /tmp/bugzilla-4.2/.htaccess ./
chown apache: * -R

Verify Module dependency & run checksetup to complete upgrade

cd /var/www/html/ 
./checksetup.pl --check-modules
./checksetup.pl