There is a replace method provided by the String class that can be used to replace a character in a String with another character. The following code snippet demonstrates this:

package demo;

public class StringDemo {

public static void main(String[] args) { 
    String str = "Hello World"; 
    String modifiedStr = str.replace('o', 'i'); 
    System.out.println("Modified String is "+modifiedStr);
  } 
}

So the above code will replace all occurrences of the letter ‘o’ in the String “Hello World” an ‘i’ and will print the following:

Helli World