How can I sing notes above high C with ease?
This is not a short answer question. First, you have to determine what is making it difficult to sing those high notes. It could be a lack of proper breath support, imprecise phonation, poor resonance balance or health issues. It might also be a limitation of the voice itself or a limitation that has developed because of the way you use the voice, even in speaking. Once you have determined the cause of the limitation, you need to find the vocal practices that help you to develop your particular weaknesses and release any tension. The best solution would be to work with a qualified teacher. You can refer to Richard Miller’s THE STRUCTURE OF SINGING for more information and exercises.