How to re-attach a keytop on butterfly keyboard?

I want to thank everyone for the helpful suggestions. I will now report the outcome. My thinking was that all the warnings about possibly breaking a butterfly or the like were more or less pointless if, as was the case, the only likely solutions were to pay to have the top end all replaced for serious money, or to junk an otherwise perfectly fine computer.

So, since I have some experience in being attentive and careful with tiny things, and since I came equipped with the insights you have all shared with me about what COULD go wrong, I took a leap and purchased an entire set of replacement key caps for my model ($19 through Amazon, lord help us all). To my relief and surprise they turned out to be the correct ones. With them I first put back home the good test key (upperleft I never use ```~~~) I cannibalized to type on the E, then fit the tiny black disk over the central switch and successfully snapped down the new E key pad. It works a treat. Replaced the worn-out A (you could see through it) and a dodgy T. There IS some extra light on the E because the masking black was hosed, but I can live with that.

The basic failure was that the little corner clips on the original (ie the Apple 'Replacement' keyboard) had sheared away.

For removing the old caps I found the safest thing a stiffish card just a tiny bit thinner than a conventional credit card, and was very, very careful to lever things from the proper location at the top.

The keyboard was replaced a couple of years ago in the keyboard replacement program, and during a call to an authorized Apple reseller/repair store nearby their service representative told me that the individual broken keys like mine cannot be replaced. This would have been the geniusbar to which I would have brought my computer to have them tell me in person what they told me on the phone. My mis-behaving E-key just rests there loosely, often walking across the keyboard or sticking to my finger, but when it is floating in place it DOES work, so I thought that just a tiny drop of adhesive of some sort, very, very carefully applied, in the center where the post and dimple SHOULD snap together but don't would be a possible makeshift. Otherwise the computer is as good as new and it would be crazy just to get rid of it over this minor problem.

Many thanks for this really helpful reply. From what I can tell, the key cap is damaged, but the butterfly mechanism seems to be just fine. In fact (my key in question is the "E" and I am typing right now with the cap remove and although it is a little tricky to get my fingertip to focus on the central boss in the butterfly, when I do it works just fine. Normally the cap just floats there, and does work, but it's not attached and will float across the keyboard or stick to my finger. When I just now put the cap aside, I found a very small black disk sitting in the boss of the butterfly, so I'm guessing that is broken off from the cap and was the part of the cap intended to secure the cap to the butterfly. I'm saving that on the chance that I could put the cap itself back together using glue, but NOT gluing the cap to the butterfly. And then trying to buy a replacement cap.

Apple replaced the original keyboard several years ago, so I can't possibly ask them to replace this one.

So one question if you don't mind. Is it possible to remove and replace a 'healthy' keycap safely, so I can see what the butterfly and an undamaged cap look like?

There was a company from which I ordered a couple of replacement caps, but they never got around to filling the order and I had to cancel.

An update. The clips for attaching the E cap have failed, and for now I have cannibalized the never-used upper-left key (``), but that one is now exposed. The main concern I have is that the black plastic shielding that looks as though it is intended to control the brightness of the backlighting is shredded away and gone. The tiny black disk flipped and disappeared, but I accidentally found it. It is not attached, but just wedged in there for now, but I think as long as the keycap is secure, it's not going anywhere. Indirectly all of your advice has helped me to do the kludge fix and I may just order some replacement caps. Cheapest is to get the whole set from Amazon I think.

Thanks!

Yes, indeed, and thanks. Those were issues I was already doing as much searching-about as possible. I think one thing that helped was the fact that the E pad was completely failed, so that I could examine the underlying butterfly and look for the clips and all of that without nosing about where I could get myself into trouble! I'm grateful that you have reminded me that not all keys even on my machine, come free from the top. I'm assuming that would be things like the space bar or other non-standard-size bits. I would definitely not risk meddling with those before searching for images. I could imagine that the tiny arrow keys could be an issue. My confidence that I would buy the correct set of replacement pads was not very high either. The vendor used the A1707 model descriptor, but the About This Mac system info does not, but uses simply the 13,3 descriptor. I had to search pretty deeply into Apple stuff before I found a chart linking these two otherwise dissimilar descriptors. I dodged a bullet on that one! I don't know whether these replacements are OEM, by the way, but I'll have to say that they look as though they might have been, and certainly snapped nicely into place. The only little lurking worry is that tiny black disk, which is just kind of friction-lying in the hole of the butterfly. With it gone (I actually lost it on the table once) the pads won't engage the central switch. Again, thanks for your patience and taking the time to help me. You kept me from doing anything stupid or destructive and saved me a significant outlay.