As I have said on other pages, you can work with what you have in a presentation. There is no need to make big changes to yourself in order to succeed, but you will need to make some small tweaks.
This is the case for keeping your presentation formal. I think almost everyone has a basic idea of how to be formal: Just imagine you're going to an important job interview, or meeting a romantic partner's parents for the first time. After that, it's mostly a case of avoiding some specific behaviours, which I will list below.
Note: This advice is for giving formal, academic presentations in class. Other presentation styles may have different levels of formality.
Try not to begin your presentation with "Hey!" or "Whats up?" of similar greetings. You're much safer with a simple "Hello".
Try to avoid anything that might sound like slang. For example: "Gotcha", "Awesome", or "Whatever". Also try to avoid lazily pronounced expressions such as "I dunno".
As far as possible, try to avoid fillers completely. A couple of "um" or "ah"s is OK, but definitely avoid more colloquial fillers such as "like".
In general, telling jokes is not a good idea in very formal presentations, but if you point out something funny in your content, that might be OK.
In terms of general style for a formal presentation, you should not seem too excited or chatty, but at the same time you don't have to be a robot. Try to imagine you're talking to someone who you have a comfortable relationship with, but you also respect deeply.
We have talked about posture on another page, but standing up straight and having good body language will help you to appear more formal.
Finally, you may want to consider dressing a little more formally than usual if you have to do a presentation. While I would never grade a student down for how they dressed, dressing nicely helps you get in the right frame of mind to give a formal presentation.