In my raspberry pi, i need to run two motors with a L298N.
I can pwm on enable pins to change speeds. But i saw that gpiozero robot library can make things a lot easier. But 
When using gpiozero robot library, how can i alter speeds of those motors by giving signel to the enable pins.
I have exactly the same situation.  You can of course program the motors separately but it is nice to use the robot class.
Looking into the gpiocode for this, I find that in our case the left and right tuples have a third parameter which is the pin for PWM motor speed control. (GPIO Pins  12 13 18 19 have hardware PWM support).  The first two outout pins in the tuple are to be signalled as 1, 0 for forward, 0,1 for back. 
So here is my line of code:
    Initio = Robot(left=(4, 5, 12), right=(17, 18, 13))
Hope it works for you!
I have some interesting code on the stocks for controlling the robot's absolute position, so it can explore its environment.
how can I use gpiozero robot library to change speeds of motors via L298N