Upgrade from Horde 5 / PEAR Horde 5 is supposed to run on PHP 5.x - 7.4 and Horde 6 is supposed to run on PHP 7.4 - 8.x. Also, the filesystem structure of Horde 6 is different. It is recommended NOT to run an inplace upgrade but backup all assets and start Prerequisites - Copy existing installation's database to a new database. At the end of the installation the schema will be changed and no longer fit for Horde 5. - Go to all Horde 5 apps, including horde base, and safe files like - registry.local.php - conf.php - prefs.local.php - hooks.php - attributes.local.php - backends.local.php - basically *.local.php - registry.d/* You normally don't need to keep these: - conf.xml - prefs.php - backends.php - attributes.php These come with the code. Modifying these was not advisable even in horde 5. Web Server Config The Web server's webroot needs to point to the /web subdirectory of the root project, not the top directory of the root project. See fresh install docs for more detail. Setup a root project git clone https://github.com/maintaina-com/horde-deployment.git Copy Horde configs Mind the different structure: old: horde/config/conf.php new: var/config/horde/conf.php old: horde/config/*.local.php new: var/config/horde/ old: horde/config/registry.d new: var/config/horde/registry.d/ old: horde/nag/conf.php new: var/config/nag/conf.php old: horde/nag/config/*.local.php new: var/config/nag/*.local.php etc Editing registry If your registry.local.php or registry.d/ snippet contains hardcoded webroot or fileroot, comment out these lines. Install Horde 6 Install the base app composer install Install additional apps composer require horde/kronolith ^5 composer require horde/turba ^5 composer require horde/passwd ^6 composer require horde/mnemo ^5 composer require horde/content ^3 composer require horde/timeobjects ^3 composer require horde/imp ^7 composer require horde/nag ^5 composer require horde/activesync v3.0.0.alpha4 etc ... see the installing chapter for more details on this step. Upgrade the database ./vendor/bin/horde-db-migrate up Check the Web UI for configs that need upgrading Goto the horde/admin/config screen. Check for all apps if there are new settings which need configuration.