You would first need to download the latest firmware from the product page of the specific amplifier. Scroll down to the bottom of the page until you get to the “downloads” section.
Once you have the latest firmware saved to you computer, log in to the web interface of the M series amplifier and navigate to the Device Configuration tab. In the bottom left corner of the page, you will find the firmware update button. If you need help on how to access the web interface, please refer to this KB article here
The amplifier will then begin updating the firmware. There will be a percentage ticker showing progress of the update.
Once it reaches 100%, the amplifier will reboot, it will take about a minute. At this time the web interface will be unresponsive. After a minute or so, either refresh the web page, or click on a different tab within the web interface and it will re-load the page.
You will be able to confirm the update by checking the revision # in the top right section of the configuration tab.