Summarize:

One way to 'get rid' of an immature reputation is simply to not let it bother you. Unless the specific content of what people think about you has serious consequences for your ability to make friends, get a job, and so on, your reputation matters only to the extent that you think it does. Also, caring too much about your reputation can, in fact, be a sign of immaturity. So, if you are overly worried, shrug it off a little bit. Ask yourself why your reputation matters for you. What are the consequences of others seeing you as immature? If they are minor and do not impede you in any important goals, consider shrugging your reputation off and focusing on other aspects of your life that are more important to you. Keep in mind that reputations are an approximate estimation of others' incomplete knowledge of what makes you, you. People (not necessarily knowingly) may spread information about you that is inaccurate or entirely untrue. Be honest with yourself. Ask yourself whether you believe you may be immature and if so, whether it might be worth changing your ways. Remember not to be too bothered by information that is not an accurate reflection of yourself, unless this information is actually damaging in a real, concrete way (i.e., it impedes with a goal that you value, such as having more friends). Take heart in the idea that just as your behaviors that contribute to an immature reputation can change, so too can your reputation.
Remind yourself that, often, reputations don't matter much. Know that your reputation might not reflect the real you. Keep in mind that reputations can change.