You can manage Merchant Fulfilled (MF) quantity through Listing Mirror. FBA quantity is retrieved from Amazon automatically and quantities can ONLY be viewed from within Listing Mirror.
First, you will need to go to the Listing Mirror Inventory Tab highlighted below
To edit the quantities, you will click under the column 'MF'. When you make any change. You will notice when you make any changes, the box under "MF" will highlight red to notate that a change in value has taken place.
When you are done making changes to your MF quantity, you will need to make sure that you hit "Update Inventory" to save your changes.
If you have any questions regarding changing your Merchant Fulfilled quantity, feel free to reach out to Listing Mirror support.