The reason there aren't a lot of general-purpose walkthroughs is that there is no general case.
The closest thing we have to a general-purpose solution is: Take the system down, do the upgrade, put the system back up. But, yes, this is often not a very good answer. It is often a lousy and expensive answer. In which case you hire engineers to build a better strategy. And I can't tell you, dear reader, what that better strategy is going to be, because it's different from case to case, and I don't know what your problem is.
And, yes, of course you shouldn't use the sloppy strategies on your financial-transaction processing app. Just as you probably shouldn't spend three engineer-weeks designing a complex zero-uptime rollout strategy for your blog comment system.