Hi,
I propose following procedure: create a copy of your Wiki-database, download the new version into a seperate folder of your webserver (don't forget all the extensions you are using in your Wiki). Copy the image-folder into your new Wiki. Copy LocalSettings.php from old Wiki to new Wiki. Change the database in the LocalSettings.php to the copy of your database.
Call home-of-new-wiki/mw-config (the adresse of the new Wiki) and follow the instructions, this will upgrade the database to the new version.
If you are using a Windows server be aware that syntax highlight extension will not work properly any more due to this problem https://www.mediawiki.org/wiki/Topic:Ugtwpe4lyuly6q98
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