To change the selling price and quantity of your NFT in the marketplace: Head to the NFT you want to modify, select "Buy > Sell / Offer > Enter the price and amount to sell".
1. If your "Sell / Offer" button is greyed out, meaning you do not hold enough amount to sell (<1). Please check if you have all of them on sale (or you do not own any), and tap on the "trash bin" and take back your offers before making any changes.
2. Changing quantity does not mean changing "issuance number of the NFT". Issuance of a NFT could only be decided during the NFT creation process, and couldn't be modified once the NFT is published.