HOW TO UPDATE FROM v3.6? 1. We DIDN'T changed database structure in v3.7. You can download new version and overwrite all files in "db/" and "files/" directories from v3.6
2. Changed or added files in v3.7: admin.php actions_admin/orders.php actions_admin/products.php config/*.php core/common.php core/files-admin.php core/pages.php lang/*.php plugins/tinymce/*.* templates/container.tpl templates/admin/container.tpl templates/admin/style.css
*.php - it means all files in directory
If You want to update from Quick.Cart v3.6. Check changes using program Total Commander. Select file in Quick.Cart v3.6 (first column) and the same file in Quick.Cart v3.7 (second column) and click in menu "Files" > "Compare by Content". You will find changes made by us.
HOW TO UPDATE FROM v3.5? You must first update to v3.6 http://opensolution.org/Quick.Cart/forum/?p=readTopic&nr=6835