Help me in solving STRJ6V2 problem

help
,
1

My issue

My code

class Codechef
{
	public static void main (String[] args)
	{
		// your code goes here
		String word = "Ocygen";
        char[] wordArray = word.toCharArray(); //why do we need to type this line
        wordArray[_] = '_';
        word = new String(wordArray);
        System.out.println(word);
	}
}

Learning course: Learn Java
Problem Link: CodeChef: Practical coding for everyone