Update wizard
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Make sure that the iTop configuration file (config-itop.php in conf/production) is not read-only,
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Point your web browser to http(s)://location_of_itop/setup and follow the on-screen instructions.
The step by step wizard to install the application is reused for an upgrade with a few differences.
The first step checks the consistency of the configuration and
the prerequisites for MySQL, PHP and optional PHP extensions. It
may fail during an upgrade as it can be more restrictive than it
was in a previous version.
For example after 2.5.0, prerequisites on MySQL and PHP will
change.
The second step of the update, summarized information already provided in a previous setup related to database connection and propose by default to create an iTop backup before the application and database upgrade.
The third step of an upgrade prompts your for several
miscellaneous information which can still be changed during an
upgrade: 
The following steps of the update wizard allow you to add modules, but you cannot change/remove already selected ones.
Configuring the CMDB
You cannot remove modules installed at initial setup but you can add some.
Configuring Service Management
In most cases, that has been set at initial setup time and can not be changed
Configuring Tickets Management
If you now wants to manage Tickets in iTop,
check the install for
possible options.
You cannot switch between Simple and ITIL
Compliant Ticket Management once a choice has been made in
a previous Setup, nor dis-activate the Ticket Management
module.
Configuring Change Management
If you now wants to manage Changes in iTop,
check the install for
possible options.
You cannot switch between Simple and ITIL
Compliant Change Management once a choice has been made in
a previous Setup, nor dis-activate the Change Management
Additional ITIL processes
You cannot remove modules installed at initial setup but you can add some.
Additional Modules/Extensions
If you have added Modules/Extensions, they can be installed but not removed, at this step of the upgrade process.
A module can be common to multiple Extensions.
Launching the installation
Once all the choices are made, a summary is displayed. You can drill down to check your choices by clicking on the “plus” button for each section.
To start the actual upgrade of iTop, click on the “Install !” button.
The progress of the upgrade is displayed with a moving progress bar.
Once the upgrade is done (here with a backup requested) you get:
Congratulation, you’ve successfully upgraded iTop!