If you are re-reviewing a manuscript (after round 1) and the manuscript of the current round has no tracked changes, you can use Microsoft Word to compare the manuscript of the current round to that of the previous round.
Before you do so, check the supplementary files to see if the author has uploaded a tracked changes version as suppl file, as we recommend.
Also, going through the author responses to the reviewers is usually sufficient to understand what authors changed - authors are supposed to provide detailed, numbered responses. See As a reviewer re-reviewing a revision, where do I find other reviewer comments and the authors' responses to the previous reviewer comments?
If you still want to see what exactly the author changed in the manuscript (although authors are supposed to describe the changes in their response), please proceed as follows.
Getting the revision from the previous round
In the Reviewer menu, click on the Archive tab (at the top) and find your previously completed review for the article in question. Go into the review and download the manuscript. This is the revision from the previous round.
Getting the revision from the current round
In the Reviewer menu and on the Active tab, find your incomplete review for the article in question. Go into the review and download the manuscript. This is the revision from the current round.
Microsoft Word > Tools > Track Changes > Compare Documents
Once you have the 2 manuscripts to compare, go Tools > Track Changes > Compare Documents, select the 2 doc files, and the result will be the same as "tracked changes".