Allow your voice to come out naturally without force, but with proper control of breathing, vocal chords and diaphragm control to support singing notes to the end.
Exercise the muscles of your vocal cords. In order to support singing notes to the end, you need to learn to coordinate your vocal muscles so you can sing in a higher range with less air.
Practice with your voice regularly to keep it in condition so it's always ready to sing. Regular vocal exercises will build your voice so that you will have a good vocal range.
Sing arpeggios regularly to practice the highs and lows of different notes. This will help build strength and vocal range.
Identify a break in your voice when singing high in an octave range. Once you identify where your voice tends to break, do exercises like arpeggios to help you move across the break successfully so that your vocal range can eventually move smoothly over the break.