It's great to hear that you're making progress on the alto saxophone! When it comes to playing high notes, there are a few things to keep in mind.
First, make sure you're using proper breath support and air speed. High notes require more air pressure than low notes, so try to increase the amount of air you're pushing through the horn.
Second, experiment with different reed strengths and mouthpiece tip openings. A harder reed and/or larger tip opening can help you achieve a stronger sound on high notes, but it's important to find the right balance for your playing style and skill level. You may want to consult with a music teacher or saxophone specialist to help you find the best combination of reed and mouthpiece for your needs.
Finally, practice regularly and be patient with yourself. High notes can be challenging, but with consistent practice and attention to technique, you'll be able to improve your sound over time.