You would have to pass down to the Word component a list of used letters (the same you pass to the used letter components, I guess). Then when you do the map function inside your Word component, you would need to introduce a condition: if the letter is part of the used letter array, display the letter, else display an empty box.
If it were me, I would create a letter-box component that takes a prop of “letter-displayed”. If the letter has not been picked yet, “letter-displayed”="", else you pass in the actual letter. Of you could have two props: visible(true or false) and letter(letter from the word)
But either way, your Word component is missing the information about what letters have been chosen so far. Once it has that, it’s just a matter of having some sort of if statement to change the display based on that list.
I can look at it more later if you want. Right now I have to leave to catch my bus.