Controlling step down converter with PIC

Hi i am building a small dc/dc converter usinf st1s10 step down module (standard synchrounous step down converter) all of those conveter use a feedback pin with two resistor for setting out the voltage . feedback voltage is always about 800mv. My question is i would like controlling the output voltage of the converter using a PIC (converting pwm to voltage reference). how can i connect my voltage reference to the converter.
i was doing before with a potentiometer before but i cant send a voltage directly at Feedback pin. i would need a virtual potentiometer!.
could i use a transistor to simulate a varaible resistor? Could i replace Rin with a transistor or ic that simulate a resistor to change the gain of amplificator.
Thanx

leave the gain fixed and add a divider after the gain stage, if you must... better still get an LED driver and use the PWM input to control the LED intensity

O sorry I will not use this for led driving, schematics is just an example of adjusting current with fixed resistor, i must adjust it with a PIC so does it exist a chip that simulates a variable resistor

ive found the answer, i will use max 5389 digital potentiometer