I had that problem for a while, but found a way to improve my range dramatically fairly quickly. In short, play scales. I know that can sound kinda boring, but it helps. Play the chromatic scale, or your favorite scale. Go as high up as you can, and then hold that note. Make sure you are in tune and have a good tone. There is absolutely no use in playing high unless it sounds good. I can play a double high C, but it sounds so terrible there is no point to it. Do this everyday, probably during your warm up, and every few days or weeks (depending on how fast your muscles strengthen) you should be able to play one note higher. After all that, and after every practice, warm down. Play down below C and some pedal tones if you can. If you get all high notes without warming down, your sound will be very bright and you will likely play sharp. Your muscles are contracted, so you need to loosen them. It's just like working out. Do that and by summer you can probably play an F above the staff. If you have any questions feel free to message me; if I know anything about trumpet, it's range.