Inventory management with local store qty product

I've my own local shop as well as online store.  Suppose my product qty. 10. in my shop I've sell 5 product in locally then how to manage(update) inventory in online store. Not possible regular update how to automatically reduce product qty.

RedHot wrote:
If you have a POS/ERP system in your shop, you have to connect it with the online store to report all changes in stock and to be updated of all online orders that affect stock. Otherwise the update has to be made by hand or by a batch process

Thank you so much Eduardo for your suggestion.. I'll try it pos/erp.