Just got to the part where you have to choose a chest to open which supposedly affects the ending when you enter the tower.

It occurred to me that I wanted to experience all 3 endings, but didn't want to complete the game 3 times from the beginning to do so.

So for the less technically savvy who want to try all 3 chests or just experiment with skills using the orb of rebirth, save their experiments and don't want to redo the entire game several times just for this:

1) Go to the point where you have to choose one of the chests at the tower's entrance and make sure you haven't used the orb of rebirth up to that point.

2) Save your game.

3) Go to your game's save folder (<GOG Games Root>\Unepic\save, for me: <GOG Games Root> is C:\GOG Games).

4) Copy the save folder (<GOG Games Root>\Unepic\save) in another place to have a clean backup.

5) Go back to <GOG Games Root>\Unepic\save, find your save game (if you have 1 saved game, it should be game_0.sav) and copy it in the same folder changing the number at the end (given that you have 9 save slots ingame, 0 to 8 should be valid). In my case for example, I had only game_0.sav so I copied it and named the copy game_1.sav.

Voila!