How can I use stepper for specific degree?
stepper.step(num)is used to rotate your motor 'num'-step. i.e. num=25, then calling stepper.step will cause your motor do 25-step.

STEPS is number of steps per revolution for your motor. It will be depending on the motor you used. For more information, you can check here.

You have to check for your motor stepper resolution. Most common motor stepper has 200 steps per revolution, which means 1 step is 360/200 = 1.8 degree. If you try Stepper.step(1), your motor will rotate 1.8 degree.    

So in your case, if you already found out the resolution of your motor stepper, just do a simple conversion task:

const float resolution  = xx.xx; // put your step resolution here
int step_degree(float desired_degree){
    return (desired_degree/resolution);}


This can be use by calling

stepper.step(step_degree(40)); //rotate 40 degree.


--EDIT--
You can check this link, since it (perhaps) using same motor stepper (24BYJ48)

Based on 24BYJ48 datasheet, it has 5.625 degree resolution / 64 steps per revolution.