Sure, I can help you with that!

Here are the steps to replace the word "YES" with a check mark symbol in Microsoft Excel:

1. Select the cells that contain the word "YES" that you want to replace with a check mark.
2. Right-click on the selected cells and choose "Format Cells" from the context menu.
3. In the "Format Cells" dialog box, select the "Custom" category.
4. In the "Type" field, enter the following code:
   
   
   Formula:

   [Green]œ”;[Red]œ– 

   This code will display a green check mark symbol for any cell that contains the word "YES" and a red X symbol for any cell that does not contain the word "YES".
5. Click "OK" to apply the custom format to the selected cells.

That's it! Now any cell that contains the word "YES" will display a check mark symbol instead.