Have you tried just connecting to your Rpi2 using net use \\systemname or \\IP Address?
net use X: \\10.125.148.238\C$ /User:10.125.148.238\administrator
Alternatively you can also manage / upload .appx files to the devices by using the web portal:
http://IP Address
Or you can also connect to the system using an FTP client to move files to / from your desktop.
As for changing the actual 'workgroup' I don't know of a command on the device that will do that for you. The reg command exists, so if you were able to locate where the workgroup value is stored you could change it then.
This post also has some similar suggestions:
viewtopic.php?f=105&t=109259
Jonathan Tanner | Microsoft | Windows 10 IoT Core Insider Preview Support | This posting is provided 'as is' with no warranties and confers no rights.