You will be prompted with a one-time pop-up notification when you open the site editor for a site that has an upgrade available. This pop-up will explain what the upgrade provides and you can decide to move forward with upgrading or ignore the upgrade temporarily.
If you have previously ignored this upgrade you can find the option to proceed by clicking the Styling button in the left sidebar of your theme editor. You will see an orange box that describes the upgrade again with the option to click Upgrade to continue.
The upgrade walkthrough may slightly vary depending on the theme associated with the site, as described in this article.