To view the changed files between current version and previous version:
Step 1: extract your downloaded package from ThemeForest.
Step 2: go to https://diffy.org. Click "Upload Diff" then browse to the folder you extracted in Step 1, open "Changelog" folder, then upload the diff file of the theme style you want to view the changed files. Ex: precia-1.diff.
Step 3: click "Diff me" and you will see the changed files.