Every time I change the battery on the coolpix 995 I have to reset date and time and all other settings. I never had to do this with my 990. How do I keep all settings , including film quality , unchanged during battery change?

Your problem is that your camera actually has two batteries. There's the obvious user-accessible one but there's also a tiny rechargeable one hidden inside the camera which recharges from the main battery. The smaller battery is designed to provide has just enough power to maintain the camera's memory of time and date settings for up to a few months when the camera isn't being used.

Like all rechargeables, it doesn't last forever and needs to be replaced to fix the problem. Unfortunately, it's not easy to get at and the camera must be partially dismantled to access it. It's been a long while since I took a 995 apart simply because there are so few left in working order; yours is nearly ten years old and it was only designed to last a maximum of five years so has done really well.

I cannot provide a link to a free servicing manual, but if you search online you should find one but it will cost more than your camera is worth. In any case, I strongly advise you not to bother: your camera may be into extreme old age (compared to it's original design life) but it still works mostly correctly. Disturbing it by taking it apart is likely to cause more problems than it solves. If I recall correctly, the internal battery is not especially difficult to get to, but is soldered in place and so is not easy to replace. It's also very easy to ruin the new battery by getting it to hot when re-soldering it back in.

The 995 was a fine camera in it's time, but like all old digital cameras is now virtually worthless simply due to the fact that it's now immensely obsolete and also due to the uncertainty of knowing what other component will fail next due to age or wear. My sincere advice is to live with the fault and continue to enjoy use of the camera while you still can. It's extremely surprising that the rotating body joint hasn't yet failed, and the fact that the sensor still works is also impressive. If nothing else kills the camera, it will likely be the sensor and probably in the near future. You won't find that modern Nikon consumer-grade models are anywhere near as well built.