Habari's upgrade process is designed to be transparent, with the only indication that an upgrade is happening being a slightly longer page load time the first time the site is visited after the software is updated.
Before making any major change in your Habari installation, it would be prudent to create a backup of your database and your installation, in particular all the files in your Habari /user directory. The /user directory should contain all files which you have customized - plugins you have installed, themes you have installed, class files you may have modified, or locale files you have installed.
There are several changes that you should take into account when upgrading.
All users will be affected by the change in theme functions and many will want to remove a potential path disclosure issue. Now that it has been removed from the Habari installation, users of the k2 theme will need to manually include it.
Theme functions no longer print content by default but now return a value. This means that you need to tell your theme to echo the returned value. For example, all themes should call the header() function, which should be changed from this:
<?php $theme->header(); ?>
to this:
<?php echo $theme->header(); ?>
You should check your calls to at least header(), footer(), prev_page_link(), next_page_link(), page_selector(), and feed_alternate() in your theme.
If you are using the k2 theme that was previously included in Habari, you should download k2 and included it in your themes directory, user/themes. k2 can now be found in habari-extras.
A path disclosure is an error that can show the directory where your software is installed, which can be seen as a security issue. To protect existing Habari installations from a potential path disclosure issue, you can modify your config.php file and include the following line at the start of the file. New installations will be protected by default.