Cisco Catalyst Stack Upgrade
Well… It will reboot your whole switch stack at once, In case you were wondering. But it has a neat feature of automatic rollback to the previous IOS XE version if something goes south with the newly upgraded switches.
The same goes for non-stacked Cisco Catalyst C9200 and C9300 switches, but the question was, and the answer is hard to find if the stack would reload members sequentially or it would just reload all members at once. The answer is of course the least good option which makes the upgrade impossible without network outage even if other devices are connected to the stack redundantly (to two or more stack members).
The whole procedure is fairly simple:
Switch Upgrade install mode
New Cisco switches are now usually shipped with install mode configuration for software installation. The other (older) bundle mode was simply a boot variable on the switch config that stated where the .bin files is saved on the switch flash and if multiple .bin files which one to load upon switch reboot.