If ILIAS was installed from SVN sources, updates are fairly easy. Nevertheless, before starting a major update, backup your system's state to be prepared for any inconvenience. If you are working in an virtualized environment, take a snapshot of your vm, proceed with the update and go back to normal operation (no snapshot) when everything works fine.
 
The update procedure:
 
Before starting the software-update, change all users to the default ilias-delos skin.
 
First change to the ilias directory on your server, then update the Ilias code.

cd /srv/www/ilias
svn up

This may take a while (some minutes) without any reaction, don't panic.
After this update database updates have to be applied. Enter the ILIAS setup (Yoururl/Setup/Setup.php) and log in. Apply all offered uptates or hotfix to the database. For larger updates, ILIAS uses revision numbers for update breaks. These suggestions should be followed to ensure the maximum execution time of the php-code isn't exceeded.
 
After the database is up-to-date, log in as ILIAS-Admin, change to the language administration and update all languages.
 
As the final step, the skins can be re- installed. Usually you have to apply changes you made to the skin to a copy of the new skin-template.
 
To update to a newer Version of Ilias use the instructions provided in the "change branch" chapter

If your system was installed from the repository at University of cologne, you can relocate to the current ILIAS repository using the command

svn switch --relocate https://svn.ilias.uni-koeln.de/svn/ilias3/branches/Release_4_3_x_branch https://svn.ilias.de/svn/ilias/branches/Release_4_3_x_branch

Modify this command to reflect the branch you use. After that use

svn up

to update.