I have trouble hitting high notes any advice?

Try aiming the air downward and make your lips as if you sucking a thick milkshake through a straw. But don't close your lips completely or no air will come out. Hope I helped!

first of all dont try blowing hard. just relax and tighten ur lips. this might sound useless but it works really well! hope u can do it!

Use your diaphragm mais and lip tension less. At the same time don't press the instrument hard on your lips.