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The buttons and valves on the trumpet have the largest impact on changing notes, but your embouchure is important too. Tighten your embouchure when playing high notes and loosen it when playing low notes.  Without your mouthpiece or trumpet, practice tightening and loosening your embouchure and notice how the pitch of the buzz changes. This effect is amplified when blown through the horn. Next, practice with just the mouthpiece and notice the siren-like sound you can make by changing the embouchure shape. Your embouchure is not effective when you don't have enough air. Sitting up straight keeps your airways open so that you can blow with enough force to produce the full scale of notes. Good posture also keeps your trumpet raised. Slump down in your chair and notice how you can't breathe as deeply as when you are sitting upright. Practice good posture when you aren't playing so you get used to sitting up straight. Your lips will get tired easily when you are first learning your embouchure. Practice holding it for a few minutes. Then practice buzzing for a few minutes. Add a little bit of time each day so you can build up your endurance. Since many musical pieces are five to ten minutes or longer, it's important that you can buzz your lips for a long time without getting tired. If you are having trouble with a particular part of the embouchure, look for videos that describe that part or the whole process. Watching someone form the embouchure may give you a better idea than reading about it. Ask a teacher or mentor if they have any videos you could borrow, or search online for videos. You'll also be able to see the way players hold the trumpet and sit. If you are in a band class at school, or take lessons with a mentor, ask them to look at your embouchure or listen as you play to see what you are doing wrong. They will be able to help you with specific problems you are facing with your embouchure. If you don't have anyone to ask, find a local instructor who would be willing to help you out. You may be able to get some quick tips without taking a full, paid lesson.
Adjust your embouchure for different notes. Maintain upright posture. Practice buzzing your lips everyday. Watch instructional videos. Ask an instructor for tips.