Replacement step by step is okay. My observations:
1. During opening there is necessary to wide sides -wings- eschew more widely - due to button cup. White gasket ring is there very tight so yeah there is need some power as I was scared that should break the Fibit bracelet. Next two hands with spacer plectrum is useful.
2. Be also careful of metal battery plate holder - this thing is very barely spotted to the mount and is easy to tear it off and very problematic to install back.
3. My battery was glued to the frame, so there is necessary to use more strength.
4. Be careful which battery you buy. I bought battery on and my piece was 2mm longer than original, so I must cut 2 plastic -spacers- between battery and -buzzer. But it fit et all. make wires a little longer than original ( 4mm+) for better handling. Longer wires also fit but it more complicated.
5. (my hint) Also is useful to put small slices of Capton, tape around soldered wires and in layers to over them to prevent shortcut.