I flashed my mouse and now it is not responding with both LEDs blinking Red.

The mouse is in boot-loader mode. Simply flashing the firmware again returns it back to normal. You can use the XM1 Configuration Software or any firmware from the Downloads page. Make sure to unzip the firmware folder if you are flashing it without the XM1 Configuration Software and run the .exe as administrator (right click in Windows Explorer -> „run as administrator“).