Upgrade for default Zen Cart installations
Upgrading ZenCart site can be simple and complicated, depending on your existing installation zencart package, if you have just the default setup without any modifications and third-party add-ons, then upgrade is going to be easy :-
But make sure to backup your website and mysql database to your local computer (for avoid any risk)
Build a new database to mysql and allow access to this new database for the mysql user that you already used for connect the existing ZenCart based site database.
Now you need to copy all data from your existing source database to this new database (i.e. newzencartdb), this process will be create an identical database newzencartdb to your existing database then we will be using the new database for the upgrade process
Download a copy of latest compressed ZenCart program files, unzipped it and upload them to a new folder (i.e. newsite_zencart) in your hosting server
After that both configure file /includes/configure.php and /admin/includes/configure.php from your existing source files and folders to the newsite_zencart folder, and then change the mysql database name to the newzencartdb name, and change the path to your store to this new folder newsite_zencart of both the configure.php file in newsite_zencart folder, make sure that everything to ensure all are referencing this new ZenCart folder and the new MySQL database, this is extremely important so that you do not mess up the existing ZenCart Site while you perform upgrade
At begin upgrade process open up any browser and go for it for example http://www.yourdomain.com/newsite_zencart/zc_install/ to start the installation process.
NOTE: You have to select Upgrade Database option instead of new installation, upgrade will keep your current database and new install will create a clean empty database
Uncheck all the steps then confirm the first step, when first step over, follow be second step, third, and so on, perform the upgrade steps one at a time so that you can isolate the issue and address them one at a same time.
Upgrade process with customizations and third party add-ons or modules
If you do have customize or third-party add-ons or module apply to your ZenCart store then you will need read and go through the release notes of the version you are going to upgrade to, and compare differences of files in your existing installation and the upgrade version, you will need to merge the different code of block or line to come out with a upgrade version pulse your customized codes pulse third party a, you may need code file comparison software or editor such h as Beyond Compare ,WinMerg, BBedit for Mac, ExamDiff in Linux. After comparing and merging files and its codes, you will then make use of this combined version of code to finish the final upgrade procedure.