Final answer:
In Microsoft Word, the 'Find and Replace' feature should be used to replace the word 'prize' with its synonym 'reward'.
Step-by-step explanation:
To replace the word prize with its synonym reward in Microsoft Word, you should use the Find and Replace feature. This feature allows you to search for a specific word in your document and replace it with another word of your choice. To do this, you can go to the 'Home' tab in Microsoft Word, click on 'Replace' in the 'Editing' group, or press Ctrl+H. In the 'Find and Replace' dialog box, type 'prize' in the 'Find what' box, and 'reward' in the 'Replace with' box, then click 'Replace All' to make the change throughout the entire document.