1.  Backup old install:

Make a backup of your current SquirrelMail directory.

For example the httpd document directory is /home/httpd/html, that your SquirrelMail install is located at /home/httpd/html/squirrelmail-1.2.6, and that your new SM version is 1.4.0. Substitute version numbers and names as required.

$ cd /home/httpd/html
$ cp -Rp squirrelmail-1.2.6 squirrelmail-1.2.6.bak

2.  Unarchive new SquirrelMail:

Make sure that you are in your httpd document directory (/home/httpd/html) and then unarchive the SquirrelMail archive (whatever the filename is):

$ tar -zxvf squirrelmail-1.4.0.tar.gz

3.  Copy important files from old install:

The important files to copy are:
A.  Preferences
B.  Config details
C.  Plugins
D.  Themes (if you’ve edited or added any of them)

A.  Preferences
First, copy your preference data over to the new directory.
$ cp squirrelmail-1.2.6.bak/data/* squirrelmail-1.4.0/data

B.  Config details
$ cp squirrelmail-1.2.6.bak/config/config.php squirrelmail-1.4.0/config

C.  Copy plugins
$ cp -Rp squirrelmail-1.2.6.bak/plugins/* squirrelmail-1.4.0/plugins

D.  Copy themes
$ cp squirrelmail-1.2.6.bak/themes/* squirrelmail-1.4.0/themes/

4.  Change permissions:

The web server must have write permission to the data directory.

  $ cd squirrelmail-1.4.0
$ chown -R nobody.nobody data

5.  Run conf.pl:

Run config/conf.pl to see the new configuration options available with the new version, as well as to verify that all of your old options are set properly.

Always save your options

6.  DONE:

That should be all!  The most important part is copying your users’ preference files back into the new data directory.  This will insure that your users will have their old preferences.