I has exactly same problem when purchasing latest M1 a month ago. Here is what I learned from my setup:
1. In order to reconfigure inputs, I had to hard reset M1 using Reset button on the back of it. In a few minutes after reset, M1 shows up on device list with "Setup" option available. This was happening only if M1 was NOT set to automatically connect upon SlingMedia Desktop application launch.
2. Trouble with 1 was when SlingPlayer Desktop app was set to automatically connect to M1 which you want to reconfigure. Resetting M1 in this state caused PC application to go into infinite loop trying to connect to M1 which was not configured. This is obvious application bug and should be addressed by SlingMedia.
3. In order to overcome trouble in 2, you have to... believe it or not... uninstall and install back again the Slingplayer Desktop application. Yes, I had to do it once because of issue described in 2. Once software is reinstalled, it shows up the list of Slingboxes with Setup option available for M1.
So at the end I learned the hard way and made sure that "Auto connect" for each Slingbox is UNCHECKED. This way anytime I need to reconfigure/reset any box I can access its Setup withouht need to reinstall Desktop software.
Hope it helps. Like it if it does.