It appears that if you click the "Upload" button in Dropbox in your web browser, and you are trying to upload a new version of a file, with the same name as the old file, then the upload process simply does not complete.
This is probably a feature to protect people from accidentally replacing files.
Here is what to do in this case:
1. First, in your browser, delete the file in Dropbox that you want to replace with your new, changed file.
2. Second, in your browser, upload the new file in Dropbox, the one with the change.
3. Third, double-check to see that file name of the newly uploaded file is now shown in the list of files in the manuscript folder in your browser Dropbox.
4. Then, on Leanpub, create a new preview. You should see the changes in your preview now.