In the code you posted above, myStr is set to the string “Jello World” and thus index[0] would be “J”.
The next line is:
myStr[0] = "H";
This attempts to change the first letter in myStr from “J” to “H” but it is invalid syntax and can’t be done that way. You need to make that change by changing the entire string at once.
You changed myStr to the string “H”. The instructions are asking you to change it to the string “Hello World”.
Also, please use the triple back tick method to paste your code in here. On a line by itself type three back ticks. Then on the first line below the three back ticks paste in your code. Then below your code on a new line type three more back ticks. The back tick on my keyboard is in the upper left just above the Tab key and below the Esc key. You may also be able to use Ctrl+e to automatically give you the triple back ticks while you are typing in the this editor and the cursor is on a line by itself. Alternatively, with the cursor on a line by itself, you can use the </> button above the editor to add the triple back ticks.