There are 3 ways to transfer files over the network to your Recalbox :
Using the Webmanager
Using a file explorer to access the Share folder of your Recalbox
Using a program such as WinSCP to access the entire Recalbox
The first solution is the easiest one, and let's you verify BIOSes, but isn't suitable for large transfers (hundreds of MB for one file).
The last solution might be convoluted, but once everything is set up correctly, nothing should be problematic.