Greedfall is a pretty challenging games at times, but it's actually very easy to change the difficulty in the game. You can change the difficulty at any time, so you don't have to stay with the difficulty you chose when you first began the game.
In order to change the difficulty, pause the game and go to the menu. Click on "Options" and then on "Game". This will get you to the place where you can change the difficulty level. There are a total of four different difficulty levels for you; Easy, Normal, Hard, and Extreme. You can probably guess what to expect from each different difficulty level.
Keep in mind that changing the difficulty level only affect combat, and the rest of the game is still exactly the same as before.
Also keep in mind that you can change at any time, and as many times as you want. If you get stuck on a boss, just lower the difficulty to beat it, then change it back up to your preferred difficulty level.
The difficulty does affect trophies and achievements in the game, so you might lose out on some of them if you lower it.