Here's an update, and some comments on what I did. I have made reference to this on the D4 strap replacement thread elsewhere in these forums.
First, I read BlueMarlin01's directions on the replacement (via the link given above in Post #8). Make sure to work on a lint and dust-free surface. I used a clean countertop. Here are the steps that I took:
1. As BlueMarlin01 says, gently unscrew the two screws (simple enough);
2. Pry open the back cover using a small screwdriver against the "ears" where the screws attach. (again, mine came off easily);
3. With the cover off, I removed the 'O' ring verrrrry gently. Do not use anything sharp to do this, it comes off easy using your fingernail.
4. If you are re-using the 'O' ring (I did), wash it with soap and water, dry and set aside. Some kits come with new 'O' ring, battery & lube.
5. Clean both edges (back cover & watch body) of salt/crud deposits. I used a toothpick and a 'Q' Tip as I did not want to scratch the 'mating' surface.
6. Replace the battery as you removed it. I used (& recommend) the Energizer 2450. See the pic in BlueMarlin01's link.
7. Lubricate the 'O' ring and the edge of the back cover. I used 'Christo-Lube MCG111', which is designed for dive gear and available at
www.orings-online.com.
8. Replace the 'O' ring on the back cover and then gently position the back cover on the main watch body.
9 Now, MAKING SURE THEY ARE CORRECTLY POSITIONED, you will need to put some pressure on the back cover with your thumb to join the two parts. When the two surfaces are snug, insert & tighten the two screws, maintaining pressure on the back cover. (The cover tends to slide out because of the lube). DO NOT OVERTIGHTEN the screws!
The entire procedure took me about 20 minutes and cost me about $11 (battery & lube). I have done about 100 dives in freedive mode, down to 66', and about six dives in SCUBA mode, down to 117' and the watch runs like, well, a watch! Wait a minute, it IS a watch, hehehe
Keep in mind, SUUNTO does NOT recommend 'user replacement' of the D4 battery. While the above procedure worked for me, and appears simple enough, it may not be for everybody. I do a lot of freediving, and the 'dealer replaced' battery lasted only 12 months. The prospect of laying out a hundred bucks every 12 months somehow didn't seem very appealing.
Your experience may differ.