When I update the plugin via the WordPress backend, I get the error "Download failed. Forbidden"

This error message indicates, that there is a problem with your Premium version license code. Please check if you have inserted it in the options page of the plugin. Check the documentation about Premium license code.

After you have inserted your license code, please refresh WordPress' update information by using the button "Check Again" in Dashboard / Updates.

This is neccessary to update the download link with the license code you just entered. Then try to update the plugin again.