Instead of adding an element, sometimes we'll want to replace an element. You can use the square bracket notation [] to replace an element at a specific index:

List<Decimal> amountList = new List<Decimal>{200.50, 300.99};
amountList[0] = 450.99;

Decimal firstIndex = amountList[0]; // 450.99 
Decimal secondIndex = amountList[1]; // 300.99

Or you can use the set(index, element) instance method to do the same thing:

List<Decimal> amountList = new List<Decimal>{200.50, 300.99};
amountList.set(0, 450.99);

Decimal firstIndex = amountList[0]; // 450.99 
Decimal secondIndex = amountList.get(1); // 300.99