Can I change this code to use a potentiometer instead of a push button
This is a simple sketch that shows how to read a potentiometer and do something with the value, namely, adjust the delay of the blinking led.

[Source: https://www.arduino.cc/en/tutorial/potentiometer]

/* Analog Read to LED
 * ------------------ 
 *
 * turns on and off a light emitting diode(LED) connected to digital  
 * pin 13. The amount of time the LED will be on and off depends on
 * the value obtained by analogRead(). In the easiest case we connect
 * a potentiometer to analog pin 2.
 *
 * Created 1 December 2005
 * copyleft 2005 DojoDave &lt;http://www.0j0.org&gt;
 * http://arduino.berlios.de
 *
 */

int potPin = 2;    // select the input pin for the potentiometer
int ledPin = 13;   // select the pin for the LED
int val = 0;       // variable to store the value coming from the sensor

void setup() {
  pinMode(ledPin, OUTPUT);  // declare the ledPin as an OUTPUT
}

void loop() {
  val = analogRead(potPin);    // read the value from the sensor
  digitalWrite(ledPin, HIGH);  // turn the ledPin on
  delay(val);                  // stop the program for some time
  digitalWrite(ledPin, LOW);   // turn the ledPin off
  delay(val);                  // stop the program for some time
}


So, in every loop iteration you read the potPin and using that value for delay.

AnalogRead gives you a value between 0-1023, which you have to adjust to give the desired value for temperature control. That is all you need.

Use map function to adjust the value. Look at https://www.arduino.cc/reference/en/language/functions/math/map/