It's not about changing your writing style, it's about refining it. You probably couldn't "change" your writing style anyway. It's a product of hundreds of things that make the way you write unique to you; tricks you learned from your fifth grade English teacher, books that inspire you, friends you base your characters on, aversions to certain topics, good habits, bad habits, your personality in general, etc.
Don't think about it as changing the way you write. Think of it as stepping outside your comfort zone. That's a good thing, it's how we learn. Try some new things. If they don't work out, try something else. If you find some new techniques that work for you, then incorporate them into your writing style.
As for your making your writing feel less formal, it may be as simple as simplifying a few complex sentences or rethinking words that are out of place in your setting.
And, as @Cyn mentioned, voice is very important. Remember that your character is not you and should have a unique voice of their own. Even if you are your own POV character, the "you" writing the story is not the same as the "you" in your story. Picture your character sitting by the crackling campfire, trying to entertain his friends with whatever crazy tale he's spinning. How does he tell his story?