Apple differentiates updates from upgrades. Updates are smaller and mostly-compatible changes, and fixes. Upgrades are larger changes, and these upgrades can sometimes be incompatible with older apps.
You're on the latest version of macOS High Sierra, and there are no further updates for that release.
You're here looking for an upgrade, to Mojave, Catalina, or Big Sur.
How to get old versions of macOS - Apple Support
Have a backup before the upgrade, as that's the path to recovery if the upgrade fails, or if some critical app won't work.
Mojave macOS 10.14 is the last version that will run both 32- and 64-bit apps. Catalina macOS 10.15, Big Sur macOS 11, and (when it becomes generally available later this year) Monterey macOS 12 will all require 64-bit apps. Check for 32-bit apps using the Go64 tool. You'll need updates, upgrades, replacements, or retirements for all 32-bit apps on Catalina and later. This includes Microsoft Office 2011 and earlier. To continue accessing Office documents, you will need Office 2016, Office 2019, the upcoming Office 2021, the Office 365 subscription, or an alternative such as LibreOffice.
And the lack of formatting for the App Store shows up for older macOS releases for some reason. What you see works, it just looks, well, unlike Apple.
