How do I backup the system and databases?, if something goes wrong, how do I role back everything?, I don’t want to have “surprises” during the process.
To backup your files, either use SSH to log into your server and the cp command to make a copy of all the files (assuming your host allows this), or (slower but easier) just use FTP to download a copy of all of your files to your machine.
To backup your database, use some database admin software (like phpMyAdmin) to download a local copy of your whole database.
Rolling back depends on what went wrong. If it is an issue with a database update you can delete everything in the current database and then re-import the backed up version. If your files get corrupted, then re-upload the backed up files.
We test all of our updates under a few different circumstances a number of times prior to release, so as long as you are using stable releases (which you are), it is unlikely that you will have database update problems. But, you never know, and it is always better safe than sorry.