To change a region's name, you need two different files in your data/text folder. One is imperial_campaign_regions_and_settlement_names.txt and the other is imperial_campaign_regions_and_settlement_names.txt.strings.bin

To get these files, you must unpack them. Check the modding tutorials section to learn how this is done.

Now once you have these files, open the .txt one and edit it to your heart's content. After you finish, save it and delete the .strings.bin file. When you start the game, it will generate a new .strings.bin file from your .txt file and your changes will appear in the game.