If you have automatically linked your catalog and stock to the Orderchamp back office, there is a real-time connection to your own B2B portal or web shop software. Thus, after a sale in one system, the stock is also updated in the other. Here you can read if we can help you with this.
If you have not made a link, only sales on Orderchamp will affect your inventory. You can find the stock at "Products" and you can adjust via "Edit". For major changes, you can get started with Excel, Numbers, Open Office or Google Sheets via the import-export tool. Also read the article about back orders.