Fundamental PrinciplesG 10

Not much to this one, pretty much what the description says. By default you can only amend your constitution every 60 months (5 years) but you can research "Flexible Principles" which cuts the cooldown period to only 18 months. Also, whenever you enter a new era and another segment of the constitution is added, that counts as a change. So in a full-length game you only need 3 manual edits of the constitution as you'll get the other 3 just by going into new eras.

You don't have to change everything about your constitution, just one change will do. I believe you can even change the same thing back and forth (i.e. change military to "Professional Army", then to "Militia" and back again) until the achievement pops. You might as well try to make changes that are useful unless you're playing a sandbox game just for this achievement like I did. I don't recommend that though, it was pretty boring just waiting out so much of the game.