PIC24 - multiple PWM duty cycle changes
Your question is so broad that it will likely be closed soon, so I don't want to spend much time on this.

However, briefly, the main PWM "value" is its duty cycle.  This is changed by writing a different value into the appropriate duty cycle register.  To figure out what that is and what restrictions there might be, you have to look in the datasheet to see what peripherals your particular PIC has, then read about the PWM module in detail in the Family Reference Manual.  The datasheet will tell you which chapter is relevant for the particular implementation of the peripheral in your PIC.

There are two types of modules that can produce PWM outputs natively. The bare PWM modules are called "output compare".  There are also more fancy PWM modules meant for more specific applications.  These are generally called something with "PWM" in their names, like "motor control PWM" for example.