How to Use the Thesaurus in Microsoft Word

Step 1: Select a word in your document. Just click and draw your mouse to highlight the word you want to look up in the thesaurus.
You can also just double-click the word to select it instantly.
    

Step 2: Right-click the highlighted word. If you don’t have a right mouse button,
you can press Control as you click the word or tap the trackpad with two fingers.

Step 3: Click “Synonyms” on the menu. Another menu will expand, displaying a few select words that have the same or similar
meaning as the word you’ve selected.

Step 4: Click Thesaurus on the menu. This expands the Thesaurus tab on the right side of your document.
The new window displays the suggestions you saw on the previous menu, and in many cases, several other suggestions.

Step 5: Click the arrow to the right of a synonym ▼. If you don’t see the arrow, hover your cursor over it first.
A brief drop-down menu will expand.

Step 6: Click Insert. This replaces the word in your document with the selected synonym.

Credit : https://www.wikihow.com/Use-the-Thesaurus-in-Microsoft-Word