In the sale page of the collection or the NFTs you have created, you must go to "Options" and, there, one of the possibilities is to select "Change Price".
When you do so, the following window will be displayed, where you will be able to see the current price and the items that are on sale as you had previously defined:
There, you can change the price as well as change the quantity of items on sale:
After modifying these values as you wish, click on "Update".
When you do so, the following notice will appear...
What you must do at that moment is to confirm with your wallet the modification you wish to make:
Once you have signed the changes in Metamask, you will receive a notice from the platform that the change was made successfully.
That's all for this tutorial! You have learned how to change the price of NFTs and the amount of collectible items for sale in the Marketplace.