If you upgrade your TYPO3 installation from one major version to another (e.g. {currentVersion - 1} LTS to {currentVersion} LTS), then the Upgrade Wizards guide you through the necessary steps to upgrade your database records or explicitly install extensions that ship with the latest TYPO3 version.
Some wizards fully automatically check whether they should be executed, while others just set a flag in the system registry if they have been executed once. This "I have been executed" flag can be reset by selecting specific wizards from the list below, so the according wizards show up as possible upgrade wizards again.