Suppose I see a monster/npc/race described in material from previous editions (likely 2nd or 3rd) and want to rework it for 5th - how can I do so?
There's an official guide published by Wizards of the Coast on converting material from previous editions of D&D to 5e, though WotC don't exactly make it easy to find thanks to the awful site navigation design. It appears to be most recently linked from the article Rules References: August 2017 - the specific PDF can be found at this link.
The first part focuses on converting player characters from previous editions, and includes DM sections about converting races and equipment. The second part focuses on converting adventures/modules, including monsters from those modules. Obviously in the first instance they always just suggest using whatever updated version already exists in 5e, but where no such equivalent exists, there are "quick conversion" rules you can follow to make a quick & dirty monster that at least has all the necessary stats in roughly sensible values to run as a 5e creature, but if you want to do a more detailed conversion it largely suggests simply recreating the monster from scratch as a 5e entity using the DMG's rules for designing new monsters.