In Left 4 Dead 2, you can change your character in single-player by accessing the player menu. To do this, press the "Esc" key on your keyboard and then click on the "Player" option. From here, you can select which character you want to play as. You can also change your character’s appearance, such as their hair color or clothes.
Changing your character can be a fun way to mix up your game play experience. It can also be helpful if you want to try out a different play style. For example, if you usually play as Ellis, you may want to try playing as Nick to see how it feels.
experiment with different characters to find the one that suits your play style the best!