Adam gave the most useless tip in the book right there, I have been playing trombone for nearly 6 years though and I can tell you that while it is cliche, simply having experience and a lot of practice is a very important thing, learning to increase range is a slow and somewhat daunting task but like most instruments, you will find yourself eventually doing it effortlessly, it might even surprise you. To simply learn to reach these notes, I suggest learning and memorizing scales in multiple octaves and trying to go as high and low as you possibly can, increasing past your level little by little as you practice. Learning to play in a very high range and doing it in a flexible way in music with good tone requires you to play the right stuff honestly. Find jazz and classical sheet music and practice through those as best as you can, they will really give you a lot of area to practice in speed, range, sound-altering techniques, and octave jumps. Also blasting out doubles and triples really improves when you visualize it in a certain way, you are tightening your embachure as well as pushing out double the air that you normally would, think of it like shooting a high speed laser beam of air into the mouthpiece. I know I am 3 years to late but I hope this helps
