This guide will show you how to change the Quform entries directly to an .xls file instead of a .csv file.
Download the PHPExcel library from this link and extract the zip file.
Upload everything inside the Classes folder of the PHPExcel library to the wp-content directory, so the files should be should be like this:
Edit the file wp-content/plugins/iphorm-form-builder/admin/admin.php, search in the file for
function iphorm_export_entries() and replace the entire function with the code from here.
Now when you export entries from the Quform → Export page they will be exported directly to a .xls file.
Note: if you update the plugin this change to the admin.php file will be overwritten.