You should use already tested and well documented libraries in favor of writing your own code.

org.apache.commons.lang3.
  StringUtils.replaceOnce("coast-to-coast", "coast", "") = "-to-coast"

Javadoc

• https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html#replaceOnce-java.lang.String-java.lang.String-java.lang.String-

There's even a version that is case insensitive (which is good).

Maven

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.7</version>
</dependency>

Credits

My answer is an augmentation of: https://stackoverflow.com/a/10861856/714112

String.replaceFirst is heavyweight

 public String replaceFirst(String regex, String replacement) {
        return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
 }

this is the fastest way

str = str.substring(0, 4) + s.substring(5);

You should show us what you have done unto this point however I am providing a hint here. Note since it is homework I will Not just give you the answer

    public  static String changeFirst(String s, char oldChar, char newChar){
    //now how to implement it 
    //play around with methods like this
    s.indexOf(oldChar);//this gets you the leftmost occurence
    //and 
    //string+char or string+ string or char+char creates a sttring
    //and try "someString".substring(a,b);// creates a substring from a inclusive to //be exclusive (0 is the first       character.) so "foo".substring(0,2).equals("fo") //f is the 0th character o is the first and the second oh is the     2th character //but isnt counted
    //next time put some effort into the questions you ask here let us know all the information and the issues you had
    }

Try using replaceFirst() (available since Java 1.4), it does just what you need:

string = string.replaceFirst("ha", "gurp");