I never resolved mine as my socket clip on the board is too loose to keep the ribbon in place. My only two options are to either slip something behind the cable to keep it in place or use blu-tack which seemed to fix some members issues on another forum.
The forum with the pictures only has the initial steps but the host for his images have removed the images so I cannot visually see the entire process.
The process starts with the two screws at the bottom. The grey base plate should come off. Then the plastic on the left should come off gently. There will be two screws holding the internals inside that need to be removed. Gently slide the internals North looking from the front of the unit. For the very first time I opened my unit back in 2005 it took a fair bit of force but when I opened it recently it was quite easy. The issue is for me that cable for the cross key had a tiny piece of cable holding it in. Upon removal that came off, seems it was glued when I sent it in for repair under the Sony recall. This has made the cable "too thin" to stay inside the slot thus making it difficult on keeping a consistent placement where all 5 buttons on the front work.
If the cable has a long brown backing to it, then it should be easy to reinsert afterwards.
Once it's removed, you should be able to slide the internals right out. To remove the battery, there will be a white plug for the cable to the motherboard that should be gently removed. A single screw will at the top left of the battery looking from the top of the internals should be removed, it will allow you to pick up the small metal base plate holding the battery. My original battery was quite well glued onto this board so I ended up bending mine upon removal. A hammer should help flatten it out.
Place the new battery against this plate and reattach the cable and screw. Then reassemble the unit in reverse of the steps noted above, so after the battery is in place slide the internals back in until there is about 1.5 cm gap allowing the cross-key ribbon cable to be reinserted into its slot. Then slide the internals right down, two screws on the left go back into place. The small plastic goes over the screws and then the base plate is reattached to the shell and two screws hold it back in place.
Luckily for me I still have the MC40 remote so I'll continue to use that to control the unit. When I have some extended free time I will disassemble the unit and see if I can get the cable to stay flat in it's socket.