You would need the Crystal Reports product to modify a Crystal Report.  Some applications have a Crystal Report editor built into the application.

Since there have been a number of versions over the years, you would need a fairly recent version of the product.

I am not sure how to answer that.  I normally do not take other people's Crystal Reports and modify them.  One issue is compatibility.  If the report was created with an older version of Crystal, if I were to modify the report, you might not be able to run it since it coverts it to the version I have.  Since I work for a government agency.  I cannot use there equipment and software to work on some else's report.