Hi,
Permission 755 is probably not enough.
For example, if the unix user of your web server is "Apache", and the unix user of your FTP server is "ftp". If you uploaded the files initially via FTP, then the files and folders will have the owner "ftp". And when you update your HikaShop, the installer will use the web server user "Apache".
The 7 in your 755 permission is for the same unix user. The first 5 in the middle is for a different unix user in the same user group. And the last 5 is for a different unix user in a different group. So in your case, it would be 5 applying, and that's not enough to allow the installer to update the file.