When a new version of TYPO3 is released, you should follow the guideline in this chapter in order to do an upgrade. Also follow any additional upgrade information carefully. You might e.g. want to skim the Release Notes on get.typo3.org to see if any features affect the way your site works.
Version specific upgrade notes
While this chapter is the general guideline to follow for a TYPO3 Upgrade, some specific information for upgrading from a previous LTS to this version is available in Upgrade to v10 from v9.
Think of active users
Think of users who might want to do any changes during your upgrading and/or fallback. Inform them before you start!
Basically these are the steps to be done to update your TYPO3 site: