Controlling a stepper motor with a motordriver and Arduino
I would like it to speed up, but im not sure how to do this.


Short answer:
From the datasheet:


  STEP IN is the input for stepping pulses. One steps is done with the
  positive edge of the input pulse.
  
  DIR/STOP/RESET input is normally used to choose the rotational
  direction.


That's it. Actually you don't need more control signals.
The motor speed depends on the step mode and the frequency of your STEP signal, which could be a PWM signal from your Arduino.
With the DIR signal you can change rotation direction.
Be aware not to exceed the motor's specified start/stop/max speed, otherwise steploss will occur.