Sorry to hear you’re having issues with the Model-I and the software update. Unfortunately, this isn’t the first time someone has mentioned this issue, nor the first its happened to me. I would suggest contacting Glorious support by way of either email or live chat, 8am - 8pm CT (1pm - 1am GMT/BST).
I had the exact same thing happen to myself yesterday, and followed the instructions of one of our guild members to resolve the issue (thanks again @zealot9802). The following link will take you to the post and the instructions.
https://gloriousforum.com/t/model-i-review-and-thoughts/12292/49?u=8-bit_bootcamp
This should get the mouse functioning again, but it will not be running the most current version of the firmware. Since the mouse will prompt another update message the next time Core is opened, I suggest declining that for the time being and following through with asking support for help. They may be able to get you the current firmware version, but it will at least show them that users are encountering issues.