Hi,
Upgrading HikaShop is like updating HikaShop.
If you have to same version number the upgrade process will just copying new core files.
The database is not touch (except for updates which can update the database schema), your configuration is kept, your products and your overrides too.
You do not require to put your joomla website off. The process is quite fast. It's a classical installation of HikaShop.
We will recommend you to make a backup before, for security purpose.
If you have edit some core files directly, the files will be overwrite (like for updates). With a backup you will be sure to be able to retrieve your old files, if you have any problem.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.