To explain my solution, I will describe my problem first.
I have the MX Master 3 (mouse) and the MX Keys (keyboard). I bought the mouse separated a year ago. I use it on two Macs and one PC, I assigned the bottom buttons of the mouse (1,2,3) to each of the computers: one (1 mac) via Bluetooth the other two (2 Pc & 3 Mac) via Unified device ( USB dongle that comes with the mouse or keyboard). Later this year I bought the MX keys and assigned my computers to the easy-switch using different numbers to what I had on my mouse. I thought it was going to be easy to re-assign the 1,2,3 keys to align and follow the numbers I assigned to my mouse a year ago but surprisingly is not as intuitive as it seems.
Solution: You can't delete or reset these keys but you can re-assign and depending on how your devices are connected either Bluetooth or Unified device first you'll need to do one of the following.
-For Bluetooth, you need to remove the keyboard from the Bluetooth preferences. In my case it was on one of my macs, then press and hold for 3 seconds the easy-switch key you want to re-assign again for your MX Keys then turn of and turn on again your Bluetooth setting to search for a new device. That's all you new device should work on the key assigned.
-For Unified device you need to uninstall the Logitech options software, re-install it, go back to the software and add a new device, press and hold for 3 seconds the easy-switch key you want to re-assign and wait for the screen pairing pop-up that prompt you to type the number on screen on your MX keys (don't forget to press enter after the number)
Note: you just need one USB adapter either use one that comes with the mouse or keyboard.
That's all it should work. I hope it helps. It took me a while to figure it out and wanted to share the solution with you.