When co-authoring a Word document online, you might need to keep track of the changes made to the document, including who changed what. The general rule when working on a collaborated document is that the change saved later is the change that is considered.
Track Changes and Comments in Word
To enable this feature, we need to turn on Track Changes.
Among the tabs on the top, select the Review tab. Choose the option Track Changes and turn it On.
Once Track Changes is turned On, MS Word will highlight all changes made in the document.
Under the Review tab, in the Tracking group, there is an option for the Markup which is set at Simple Markup by default. You could choose the option of your choice from the drop-down menu. Simple Markup show the position in the document where the changes were made with a red line, No Markup hides the indicator, All Markup shows all changes with multiple indicators of different color and Original shows the original document.
The Show Markup option helps you decide the type of revision you wish to check. Eg. Comments, insertion, deletions, etc.
Lock Tracking
Suppose you are the admin of the document and need to make sure that the editing on the document is a fair process. You would like to track changes, but even more, you wouldn’t want anyone else to turn off the option for tracking the changes. 
When you click on the downward pointing arrow on Track Changes, you will get an option Lock Tracking. This could be used to set a password, such that no other co-author of the document could disable the feature.
Accept or Reject changes
You could accept or reject changes made in a document as follows:
Under the Review tab, click on previous or next to reach the specific change you need to accept or reject. 
Then click on Accept or Reject to do the needful. If you need to accept or reject all changes to the document, click on the downward pointing arrows corresponding to the Accept or Reject icon and select Accept all or Reject all.
Change Username and Initials
The username and initial of an author of a document is mentioned at the top right corner of the screen. This is even more necessary when co-authoring a document, to be sure of who made what changes. The username and initials are asked when installing Office on a system. They can be changed as follows:
Click on File and then on Options. 
In the General tab, you would find the option to change the username and initials under Personalize your copy of Microsoft Office.
View or Delete a comment
MS Word has an option to create comments inside a document. The comments appear in form of a balloon and you can click on the balloon to view the comment.
To delete a comment, go to the Review tab and check the Comments group. Clicking on Delete will delete the comment which is chosen at that point of time.
Hope this helps!
