First of all backup your current website’s version (files/folders/database)
Supposed you have ZenCart on yourdomain.com pointed to /home/USERNAME/public_html
So, we will upgrade your ZenCart to /home/USERNAME/public_html/shop to prevent any issue with your current installation.
Create sub domain shop.yourdomain.com and point it to /home/USERNAME/public_html/shop
Create if needed shop folder under /home/USERNAME/public_html/
mkdir -p shop
Download the latest version
wget https://sourceforge.net/projects/zencart/files/CURRENT%20-%20Zen%20Cart%201.5.x%20Series/zen-cart-v1.5.5f-12312017b.zip… Read the rest