I could not find it in the Docs and discussions 

Generally speaking I prefer replacing (not just overwriting) the vendor folder and the Elgg core bundled plugin folders in the mod directory plus checking for differences in the files elgg-config/settings.php and and .htaccess that might need to be resolved. In case of an update from 2.3.10 to 2.3.11 a comparison showed that there are no files in 2.3.10 that are no longer there in 2.3.11, only a few new files in 2.3.11 and then only changed files. So, when updating from 2.3.10 to 2.3.11 you could just overwrite the mod and vendor folders and overwrite composer.json and composer.lock files in the main folder and it should be alright. Nonetheless, a backup of the database should always be made first before upgrading (and making a backup of the install folder and data directory regularly is a good idea, too). After overwriting the files you would run upgrade.php (or clicking on the Upgrade button in the admin area) and you should be done.

http://learn.elgg.org/en/2.3/admin/upgrading.html

Very helpful.

Thank you very much