I have just recently changed 6 of them, which shown at step 8 at this guide. Now it works as good as new.
You can find them as “tact switch” (or tactile switch) at online for very cheap prices. Dimensions are milimeter, 6x6 mm and button height is 4.3 mm. i think these are shortest of 6x6 button family.
Search at Google for “6x6x4 tact switch”. For referance:
Here is how it looks after i changed tem: https://imgur.com/a/DhemkSh
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