You have two different options for price and inventory management in the E-Commerce Configuration
Option #1 - User Interface
This is a great option for changing prices of one product at a time and dealing with less than 20 products. For detailed instructions, Click Here
Option #2 - Import csv File
Update pricing regularly by importing a structured csv file into the File Center. This is the way to go if you are looking to update quantities of several products on a regular basis. Recommended for anything more than 20 products. For detailed instructions, Click Here