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So you want to update your Vanilla software but are afraid of fatal errors. Not to worry. You just might not have read the documentation or understood it.
1.Make sure your Host has enabled the latest php version on your server, that would be PHP 6.6 or higher. And that mbstring module is enabled. It should have been if your forum worked before.
2.Upload the newest version of Vanilla folders one by one overwriting the old ones via ftp making sure you don't overwrite the conf folder. Instead overwrite the default files in the conf folder one by one. If you have created customized themes, do not overwrite the themes folder. Also do not overwrite the Uploads folder. That folder is empty by default. So you don't need it for the Update because you already have one.
3.Once you have uploaded all of the files and folders, you will have a fatal error, Something has gone wrong ! . You need to run yoursite.com/forum/utility/update and subsequently after you get a success! , run utility/ structure which will take you to the login page. Once you log in it will take you to the database structure scan . Scan it and repair if needed. Some of the info that remains is not a problem only informational.
Your forum should be ready to go as it was before. You may need to adjust some things in your theme or plugins css. The new Vanilla relies heavily on scss and it will block many css rules from plugins that once worked. You will have to add those rules to your custom theme. But most everything will work as usual. It can't hurt to carefully delete the .ini files from the forum cache.