Pi4 to Pi5 migration

What's the easiest way to migrate from Pi4 to Pi5? 

In this case I've a Pi4 running from a USB mechanical
hard disk. What I'd like to do is upgrade the existing
system OS to boot either Pi4 or Pi5, but I'm wary of
unintended consequences. The disk is fairly new and so
far has been reliable, so there's no strong incentive
to replace it.

Thanks for reading, and any suggestions.

bob prohaska