I just wanted to add a correction to BennyFish's solution above. You actually have to hold down the Tone and Tap buttons together and then turn the amp on. This echoes what Triryche posted above. That will reset the amp so you can update it. It may have previously been the master volume and Tone buttons, but at least as of May 2020, it's Tone and Tap. Also, I used a USB cable for the update (running from a Mac). Probably the most reliable way to go.
After you update the firmware, via Line6's updater app, the Amplifi 75 will automatically return to its normal starting state. Hope this helps.