You can update the stock in two ways. From the List of Payment Pages:
Navigate to the list of Payment Pages and click on the relevant page. The items appear on the top of the page, on the right-hand side.
Click Update Stock and enter the number of units available for sale in the field.
Click Update.
Using the Payment Page Edit Mode:
Navigate to the list of Payment Pages and click on the relevant page. Click the edit icon.
In the Payment Details section, go to the price field and click the edit icon.
Enter the number of units available for sale in the field.
Click Add.