Read the system upgrade thread and understand what is being updated and how it relates to your system. If you are not comfortable with what is being upgraded then WAIT for a couple of days, reading the thread for issues experienced and solutions to these issues. Let others perform the upgrade and essentially beta test this process for you. If you are new to Manjaro and/or Linux in general I would recommend you wait, read and learn anyway.
Before actually applying the update, BACKUP your system. I would recommend creating a disk image of your system disk using Clonezilla, but there are other options available. See threads in this forum covering what people use to backup their system. Whatever software you use to backup, test that the restore process actually works.
I would recommend updating from CLI instead of GUI, from this thread you see there are many ways of using
pacman-mirrors, it comes down to personal preference.
Personally I don't run
pacman-mirrors each upgrade. All my local mirrors are very quick, I find my fastest local mirror at the top of
/etc/pacman.d/mirrorslist, check it has synced using the provided link in the upgrade thread, and if so I simply run
sudo pacman -Syu.
The keyring update commands are useful to know if something goes amiss, but you should not need to run them each time.
I don't explicitly clean my package cache after each upgrade, thus keep the latest three version of all installed packages. Unless you are experiencing root file system space issues there is no need to clean your cache after each update. If you do clean your cache then do it after you are sure all system components are working properly, meaning you won't need to downgrade anything. Even then I would recommend keeping at least two versions of each package.