Anything relates to Sitecore upgrade process
Every release since Sitecore 6.0 in 2008 has built on the core strength in web content management to offer a platform that lets you manage the entire customer experience in an omni-channel, personalized manner. But for marketers who want to take advantage of the clean design and customer data that enables context marketing, an upgrade to Sitecore® Experience Platform™ (XP) 9.0 is required.
Beyond getting access to new features and ensuring that your mainstream support is still active, upgrading is a great time to think about whether your content and personalization strategies need updates, too.
Every Sitecore version comes with an installation and upgrade approach with proper documentation detailing the steps to do it:
https://dev.sitecore.net/Downloads/Sitecore_Experience_Platform.aspx
