There are some additional options at the Review and Upload stage if a proof has previously been uploaded.
The default selection to upload again will only update your Non-Design Data. This will update your Title, Subtitle, and Logo of your Proof.
The toggle to the right will allow the current Proofer subproject to be overwritten. This should only be done in cases where there were mistakes made on the splash page, designs, or slideshow uploaded to that proof. Any comments that were associated with the Proofer subproject prior to this will be deleted when overwriting.
In short, if you choose Update Non-Design Data Only, it will update the Title, Subtitle, and Logo. If you toggle Overwrite current proof and Export and Overwrite, it will update everything and delete the previous designs and comments.
Please submit a ticket here with any questions.