If you're on the MyWikis Basics plan, you'll always be on the latest LTS version of MediaWiki. When a new LTS version is released, create a support ticket to let us know if you want to be one of the first to be updated.
If you're on the MyWikis Pro, Corporate, or VPS plans, you can change your MediaWiki version to whatever you'd like. The only restriction is that the MediaWiki version needs to be compatible with PHP 7. We'll update your wiki to the latest LTS version whenever possible unless otherwise specified. When a new LTS version is released, create a support ticket to let us know if you want to be one of the first to be updated.