When your client asks for changes you get an email, and the project shows asked for changes with their note and comments.
Upload a new version
On the project page, click the upload icon on the file you've revised and choose the new file. It becomes v2 of the same item — the earlier version and its comments stay in the history.
Do this for each file you've changed. New files can be added as usual.
Send the next round
Click Send round 2. Choose What's changed and only the new and updated files go to your client. Their earlier comments stay attached to the versions they were made on, so nothing is lost.
See where they marked
A comment with a number was marked on the file. Click Show on file next to it to open the file with every mark from that round, numbered to match the comments. Click a comment to jump to its mark, including the right PDF page or video moment.
Reply to comments
You can reply to a comment from the project page with Reply. Your reply is shown to your client on the review page.