Compare and merge two versions of a document

Compare two versions of a document

1. Open one of the two versions of the document that you want to compare.
2. On the Tools menu, point to Track Changes, and then click Compare Documents.
    

   Note: If the Compare Documents option is not available, the document might be protected.

   To unprotect a protected document, on the Tools menu, click Unprotect Document.

   Src:  https://support.office.com/en-us/article/Compare-and-merge-two-versions-of-a-document-f5059749-a797-4db7-a8fb-b3b27eb8b87e