HOW TO UPDATE FROM v3.0? 1. We DIDN'T changed database structure in v3.1. You can download new version and overwrite all files in "db/" directory from v3.0
If You want to update from Quick.Cart v3.0. Check changes using Total Commander. Select file in Quick.Cart v3.0 (first column) and the same file in Quick.Cart v3.1 (second column) and click in menu "Files" > "Compare by Content". You will find changes made by us.
HOW TO UPDATE FROM v3.x? You must first update to v3.0 and after to v3.1 http://opensolution.org/Quick.Cart/forum/?p=readTopic&nr=4958
Maybe there were some changes in file container.tpl in Your skin and You replace it by default from QC 3.1. Try to compare an old file with default from QC 3.1.