If it will not auto update in WordPress, then ….
You may need to do the update manually (after downloading from your account area):
Manual installation instructions:
1) Deactivate S&F in WP Admin Plugins page (do not delete, otherwise you will lose your search form)
2) Via FTP, delete search-filter-pro folder from the wp-content\plugins folder on your server. If you wish to keep the folder, just in case, do NOT rename it, instead move it UP a folder into the wp-content folder.
3) Unzip the file that you have downloaded.
4) Upload the search-filter-pro folder to the wp-content\plugins folder on your server.