I created a new db and successfully created all tables.
And I clicked /install/upgrade.php , it shows
"This upgrade has been completed. Please remove the entire install directory"

I haven't pointed my old db and run any altertable scripts , why it shows upgrade completed ??

What's the correct steps to upgrade from 7.2.2 to 7.3.1 ?

You'll want to follow this guide.

You don't need to create tables unless you're doing a new install. It sounds like you've chosen the New Install route, since you said it created all tables, which is why it's showing upgrade complete.