Authors are encouraged (but not required) to upload tracked changes versions of manuscripts as supplementary files.
Usually, going through the author responses to the reviewers is 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 see I'm re-reviewing a manuscript revision without tracked changes, how do I know what authors changed?