At present, Topdser can't support modify the number of products on Topdser. But, you can do it on Shopify.
After an order is paid, you can edit it by adding items, removing items, and adjusting item quantities on Shopify. Edit an order if a customer wants to change an item or if you need to add or remove an item. Here are the steps:
Go to the Shopify admin > Orders page, choose the order you need to edit and click Edit.
From the newly opened page, choose the option you need to do. This example is to change the order quantity from more to less.
Then, enter the number and click "Done".
Once you finished, don't forget to click the blue "Update order" button.
This is to ensure that your modified order information can be synced to Topdser in time.
Go to the Orders page in Topdser, you will see the order information updated on Topdser, and click on the blue “Order Products” button to purchase product(s) from the supplier.
Don't hesitate to contact usif you have any questions!