To multiply an NFT is to mint more tokens of the same NFT
You can only Multiply NFTs that you have minted, and not all NFTs that you own.
On the navbar go to Create and My Stores
Find the NFT you want and click Multiply
A modal will pop up in the same page. Here you can input the amount you wish to mint.
After clicking Continue you will be redirected to your wallet to approve the transaction.
Once you approve, you will see the success page and your tokens are minted.