If you are planning to change FBA item into FBM (Fulfilled by Merchant) , please follow the below steps from Selro. It is advisable to perform this action from your Selro inventory so that you can manage the stock levels for those converted items. However most recommended way is to maintain two SKUs one for the FBA item and one for FBM item.
a) Please go to Inventory ->Channel Products and search for your FBA item that you would like to convert to FBM item.
b) Click actions menu and select 'Change to Merchant Fulfilled Item' button.
c) This will change the FBA item to FBM so that you can manage the stock level for that item.