how to increase quantity of same product in woocommerce plugin

Wamiq,

Are you talking about increasing the product in the back-end, or on the Cart page? To change stock levels, edit the product and go to the Inventory tab. Click the ‘Manage Stock’ option to enter a stock level.

As long as the product does not have the Sold individually option selected, users can purchase multiple quantities of a product, either by selecting multiple quantities on the single product page, or the Cart page. If you don’t have this option, check for the Sold individually option, then see if your theme is not displaying the Cart page properly.