Click the Upload button on the "Review Materials" section of the review summary screen. If the file or changelist can be removed, meaning if there are no comments or defects linked to the file or changelist, there will be a [Delete] link to the left of the file or changelist. Click the link to delete the file.
If you do not see the link, you will not be able to delete the file because it would orphan comments.
Note that you cannot delete a single file from a changelist, only entire changelists.