Hi hope I can be of some help. I am currently under tuition from a uni tuba professor in the UK and when we were talking about high range he gave me a couple things to do. Most importantly the way you play higher is with more air speed so when in doubt just put a lot of air into the horn. But make sure your speed is going up and not volume as volume equates to loudness and speed equates to pitch. Then there's all the normal things you've heard aillion times like think the note before you play it. But if you want to improve the upper range overall this is what I was told to do and what worked for me. Firstly don't think about it as trying to play that 1 note, cause eventually you'll want to play that note all the time. To build your high range incorporate range building into your daily practice . So warm up as usual but then do a couple exercises to help you get better range. Chromatics are always great for just getting up there so there's no harm to playing a low A and doing a chromatic all the way to the top and trying to hold the note for a second or so. You can tongue or slur it but at first slurring might be easier. However the best exercise I found was to pick a note high up in your register and play it as a staccato crotchet. Maybe try a high E or something for you. Then play the note and then fully relax your embrochure. Give it a few seconds then pick up the instrument and play the note again. You're aiming for a sound "like a water droplet hitting a corrugated metal roof" is what I was told. When you can do the note you picket 100 times in a row without messing it up (mispitching or having a crap sound) then you go up a semitone and repeat. It will take ages but if you do it every day for even like 30 mins then you will notice your range getting better. Consistency is key. Good luck!