Ok, after removing the rear cover and disconnecting the battery you will have this (obviously this would be the repairer doing this)
Then remove the support ring which also houses the button pressure pads:
This will leave the mechanical buttons expose internally - see the little yellow circle:
I believe fitment is achieved by removal of faulty button and then the little spring and washer are applied to the new button stem as a “one way” push-on.
If necessary the mainboard can be very carefully removed if needed but special attention will need to be taken where the antennas connect to the body and the cables detached and re-attached with caution:
There is also the possibility that the button pressure pad itself has collapsed:
If this is the case another spare part will be needed - the button pad/mainboard support ring.
Sadly this will be unknown until the watch is dismantled.
Ask the repair guy to be careful when removing the rear cover as this battery will also come out and has to be unplugged.
Care must be taken to re-assemble with the polarity in the correct way - but it’s not too hard.
This is one of two I have and I pull the main boards an swap things around all the time.
But - I really must NOT recommend that you try this yourself !!
The whole operation can be done in about 40 minutes by someone who repairs mobile phones.
This is why I suggest it.