Method 1

In reviewer mode, you can find new revision button at the bottom left. This will show the uploader modal, where you can drag and drop files.

Method 2

Another super simple way to upload a revision is directly via the thumbnail as shown in the image below.

https://cdn.elev.io/file/uploads/YCivSR8Fm6JjATHHNiKVQnFkJ0ucXiR5__THw6ZhwLk/mi58v8DZDSOdKV7N-Gs87jC3OyNCN6qpELq6Wzm9Ko8/gv-rev-gvg.png