In your admin side navigation menu, click Inventory:
​

 Click Products:
​

Enter name of product in Product Name box and press Enter:
​

Find the item that you want to change the quantity for and then under the Total Qty column, click the number (default should be 0):
​

Increase the Qty(quantity) by clicking the up arrow for your card's condition:

Click Save Changes on the bottom right of the screen: