You don't. Once adrenaline starts to flood your system, there is no way to lower it except to let nature takes its course. Adrenaline is excreted mostly by your subconscious mind. If you are afraid or excited it automatically triggers the release of adrenaline. Having a clear head is not the same thing as not having an adrenaline rush. You are suppose to keep a clear head even with it. The point is to be functional when in an emotionally charged situation.
The idea that you are going to be calm, cool and collect with an even heartbeat when faced with someone trying to do you serious harm or kill you is a pipedream. Unless you are a psycho, that is not going to happen.
Don't get excited. Much of it is a matter of how you approach a fight. If you fight because it presents itself and your goal is to get out of it, which does include running away as soon as you get the chance, with as little injury as possible it is easier to stay calm.
If your goal is to make a show of this, in front of all your neighbors and your friends and there is no other option for you than to look good and teach the other guy a lesson so you can be a step up on the pecking order in society/school, now you got a whole lot more to worry about, so more adrenalin will be dumped. Your ego is what messes everything up. If it's big you got more to lose in a fight.
Spending a great deal of time training with others and eventually full sparring. Doing this against bigger guys, smaller guys, better guys and guys who train different styles, or boxers, will help. Once you get past the getting hit hard part by all kinds of different people it should be easier. But it may never really go away many pro fighters of any fighting art say they get nervous before all fights.
Train to make every reaction natural without hesitation and doubt. Just like walking or mowing the lawn you don't think about it at all.
Make your everyday body and mind your fighting body and mind as Miyamoto Musashi said hundreds of years ago
Adrenaline gets "spent" when you're in a fight. For your own general health, it's not a good idea to have high adrenaline levels frequently. Adrenaline is supposed to help you get out of a dangerous situation, so don't abuse it.
You can't and you wouldn't want to.
The adreline is necessary in a "life or death" situation.
What you can learn is the ability to control your responses to the adrenaline surge.
A martial art or boxing with lots of sparring may help, as will talking to the seniors at the club; all of whom will have been through the same and will have learnt to control their responses.
Learning a martial art can help you stay relaxed during combat.
You don't.
