How can i expand my diaphragm to sing longer and better??

I guess you can try to breath in .. hold and release slowly. Repeat after. This is what my music teacher told me, cause I use alot of air when playing my instrument..
Make sure you are breathing from your stomach btw.

try using a type of breathing called a "tire tube" breath. Instead of just comming from the front of your stomach, the breath also comes from the sides and the back. This helps in creating a longer, more sustained breath.

the more you sing, the more you train, so that singing itself becomes the exercise. Having thrown that aphorism in your way, there are a couple of easy exercises that will help you focus your concentration on your breathing.
Hiss like a snake. You'll want to have the second hand of your wristwatch in good view. Keep your molars apart, and the tip of your tongue on the inside surface of your lower front teeth. As you hiss the air out ( time yourself) you will feel the muscles of your belly, flanks, and back start to get into the game. As the one woman wrote "tire tube" you want to encourage this feeling of expansion around the middle as you are exhaling. After about 20 seconds, your body is going to start insisting that you breathe again. You don't have to, not really. you have lots of air left inside you.
don't allow your throat or your abdominal muscles to collapse or tense up on you as you hiss for 2 or even 3 seconds more.
After you gaspingly inhale, wait a minute before you try again. 3 repetitions at the most. After a week or so, you will find that you start adding a lttle bit of time, and a great deal of sturdiness to your use. the sense of panic "Breathe now!"
will disappear, simply because you know that you can go a couple seconds longer. This confidence builds up in and turns into a couple more seconds.
A point on terminology: we don't use just the diaphragm to breathe with, altthough it is the largest of all the muscles we do use. although the term "diaphragmatic breathing" is more or less synonymous with good breathing for singing,
there are at least 8 more muscles groups that we coordinate into this feat, so you won't be just "expanding your diaphragm", you'll be training your whole body.
Best wishes and keep on singing