Brass instruments have a ton of power, but they are heavily lacking in range. Where a flute can easily play up to three octaves, Play just an octave and a half can be difficult for many brass instruments. The key is in the embouchure and the air you use. You want to tighten up your lips while also blowing air at a high velocity. That is, unfortunately, easier said than done. Practice your scales, and make sure you warm up plenty before you go for those high register notes. Not warming up beforehand will tire your chops out fast and potentially cause some split lips. If things don’t work out, don’t sweat it. The more you play, the better your embouchure gets, so just keep at it.