Go to your game folder, and find the history/countries folder. These are the files you will need to edit.
To get the province ID, if you know the province name you can get the ID from the localisation/text.csv file. Search for the name and you'll find the ID on the left. Or, you can boot up the game, open the console and type the provid command. Hover over the province and you'll get the ID number you need.