Bug fixes to v5.1 here: http://opensolution.org/download/Quick.Cart/bugFixes.html
Skins and plugins to v5.2 Most plugins works correctly with new version.
HOW TO UPDATE DATABASE FROM v5.0 or v5.1? 1. We DIDN'T changed database structure in v5.2. You can download new version and overwrite all files in "database/" directory (without subdirectories) from v5.0 or v5.1
Check changes using Total Commander. Select file in Quick.Cart v5.1 (first column) and the same file in Quick.Cart v5.2 (second column) and click in menu "Files" > "Compare by Content". You will find changes made by us.