IOS Upgrade of C9300 switch, reload one at a time.

[zarjer]

We are planning to upgrade our C9300 switch which are connected to the routers going to the internet. Current version is 16.6.4. We need to upgrade to 17.3.3. Since it has redundant etherchannel connections going to FW and Internet routers we expect failover. Is it possible to reload the switch one at a time so the other switch is UP and internet traffic will not be disrupted. Please see attached diagram for reference.

[Damien Miller]

It is not possible to do what you are proposing with a 9300 without breaking the stack. Some models in the past supported a rolling stack upgrade, but this was dropped. You will be best to do a traditional upgrade with full reload.

Once on 17.3.2+, this does get slightly better though with the addition of the extended fast software upgrade functionality. The 9300 wasn't intended to provide hitless upgrades like you want, splitting the two switches could potentially provide the functionality you are after but would require a redesign and requirement evaluation. 
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-9300-series-switches/216837-extended-fast-software-upgrade-on-cataly.html

[balaji.bandi]

Short Answer NO, you will have down time when you reload stack.

if you looking in the future, better use Stackwise virtual so you get ability to upgrade each on in the future.