To replace an existing value at a specific position in ArrayList in java, use the set() of the ArrayList.

public set(int index, Object obj): this method returns the element that was deleted.

import java.util.ArrayList; 

public class ArrayListReplace {

 public static void main(String[] args) {
 
 // Create an ArrayList< Integer> 
 ArrayList< Integer> aList = new ArrayList< Integer> (); 
 
 //add digits to ArrayList
 aList.add(1); 
 aList.add(2); 
 aList.add(3); 
 aList.add(4); 
 
 System.out.println("ArrayList "); 
 for(int e:aList)
 System.out.println(e); 

 aList.set(2, 33); 
 
 System.out.println("ArrayList "); 
 for(int e:aList)
 System.out.println(e); 
 }
}

Run:

ArrayList 
1
2
3
4
ArrayList 
1
2
33
4