You should always backup your database before performing updates, as it says on this page https://www.advancedcustomfields.com/resources/upgrading-v4-v5/. It will be updated all posts, postmata, usermeta, and options values associated with ACF. Also if you read the page I liked to there are some minor changed to coding that may be required, I’d read that page several times and take a look at your site code and have a good idea what you’ll need to modify before you start.
While nothing should go wrong and it should not bring your site down, there is always that possibility when performing an update. If you have things backed up then you can always revert the site to get it back to way it was.
If you try the upgrade and you can’t get it to work, submit a support ticket https://support.advancedcustomfields.com/new-ticket/
