I got an upgrade notification in my email I click on the link below
Upgrade using our easy one-click browser upgrade
it bring me to mydomain.com/admin/upgrade
I got error of
This page isn’t working
mydomain.com is currently unable to handle this request.
HTTP ERROR 500Weird! Try running sudo /var/discourse/launcher rebuild app on your server – it most likely will fix your issue, and also update Discourse in the process 
Will this reset everything? What’s the impact?
It will re-create the Docker container with Discourse, but keep all your user data (which is stored outside of the container). You should expect about 5 minutes of downtime, nothing worse
Sorted it out, I need to log in as admin then only able to access that page 
This is a legit bug @tgxworld, I get a 500 error on discourse.codinghorror.com when in incognito mode, attempting to access the /admin/upgrade – and that is on latest too.