If you want to change an Amazon Merchant fulfilled item to an FBA item you will need to do this directly in your Amazon account.
Once you have updated this product in your Amazon account you can then redownload this product into Selro.
To redownload your product please follow the steps outlined below.
Please select Inventory-> Inventory & Listings-> other actions-> download from sales channel-> Download by item ID.
Please watch the video below for a step by step tutorial on this process.