How to edit manpower and how to change it, are two different things. Sporrally is correct. Simply going to the the manpower screen where you can view each of your individual pops and clicking on one allows you to change it.
If you want to edit it, you can go into the save file form the game and find one of your soldiers in one of your provinces and you can edit it from... say... 0.059 (which is 5900) to say... .990 (which is 99,000), that makes more of an impact on the enemy than beating a few plowshares into swords.... of course, in my opinion its cheating... but that's how you edit it.