I had a cracked button and started researching how to replace it. Thanks to
@minedelta covering the button assembly on the OP, I was able to find out that just the lens is replaceable. The lens manufacturer, EAO, has a number of different buttons, lenses, etc and it wasn't as straightforward finding what was compatible. So I reached out to them and they shared this list:
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I ordered the original ones, 84-7111-500 (Green) and 84-7111-200 (Red) from Digikey. I guess you could get creative and have different color buttons.
I also asked EAO if there were any replacement instructions and they don't have any formal documentation. They told me to use a thin plastic stick to pry the lens out. I only had a metal tool, inserted it between the lens and the bezel, and slowly pried up and around the lens. It didn't take much effort for the lens to pop out.
Unsurprisingly, they were filthy. Putting the new one in was as simple as pressing them in. To my surprise, tactility was improved drastically on both buttons (previously the green button was less clicky and more thuddy). It was clear the grime build up contributed to the loss of tactility.
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It is a very inexpensive fix that doesn't require opening up the unit. Hope this helps other DM owners.