The text for this can be found at https://smallbusiness.chron.com/reset-microsoft-words-spelling-grammar-check-75554.html
The text from that article is just below.
An image I have created may be more helpful for visual learners (like me!)
How to Reset Microsoft Word's Spelling & Grammar Check
Reset for Single Document
1
Open a document in Word and enable Spelling and Grammar check on the document by clicking “Review” in the top navigation ribbon, and then clicking the “Spelling & Grammar” tab. Change or ignore misspelled words, as desired, when the Spelling dialog box displays a suspected misspelling. Spell check the entire document.
2
Click the “File” option, and then click “Options” once the spell checker has reached the end of the document.
3
Click the “Proofing” option, and then click the “Recheck Document” option. A warning message displays indicating that the operation resets the spelling and grammar checker so that Word will recheck words you previously checked.
4
Click “Yes” to continue, and then click “OK” to close the Word Options box. Spell check rechecks the document.