Just like everything, you have to practice, practice, practice!

Train your vocal chords. Start by singing "Do (dough) Re (ray) Mi (me) Fa (faah) Mi Re Do" in a lower octave. Each word go up 1 note until you reach Fa, then lower 1 note as you go back down to do (dough). Repeat, but start 1 note higher on do (dough) than you did previously.

Repeat this until you reach a note that you can no longer make cleanly.

Do this 1-2 times a day for a few days, and you'll hear yourself able to reach higher and higher notes.