To update extension from Backend side, you should follow below steps:
1. Register a Download ID from: www.joomdonation.com/download-ids.html and enter it in Configuration page
2. Publish the Joomla Installer plugin: Joom Donation Update package.
After that, you will able to update extension from Backend side.
With the issue in Joom Donation configuration, you can submit ticket on category Joom Donation and provide your site url with super admin account. I will help you to solve it.