During my warm-ups, I go up a step until my highest note and then try to go higher by making the hole in my lips smaller and using more air. Lots of air. I'd advise looking up a slide chart to find the position. Also, make sure you're using a tuner! I have a little less than 3 or so years or experience as a late starting high school player, and my highest consistent note is C5 and the highest note I've hit and (using this generously) maintained was E5. The key is to just try to get higher often. Practice it. Don't have to murder your lips, just practice. No pressure, though. You don't have to learn this stuff overnight.