How can I sing higher notes?
Mostly by not forcing them. Pushing more air and volume at a high note tends to make the throat squeeze and the voice crack. What extends range is gradual: slides and scales that go a little above your comfortable top on a light, quiet sound, lip trills and straw exercises that keep the effort low, and letting the voice move into its lighter register instead of dragging the heavy chest sound upwards. A few minutes on most days works better than an occasional long push. Stop if your throat hurts or your voice goes hoarse.