I built a small summer home and I used the Hardy Board or something similar make of a composite, it had a 25 year warranty, well the bottoms of these boards are beginning to soften and crumble where the water splashes up on to the siding. Does someone have ideas of what and how I could replace the bad part up about 18 inches to 2 foot. What I have now is 4X9 vertical panels with a 2 inch wide strip of the same material every 16 inches running from top to bottom to make it look like a board and bead effect.
Hope you can understand what I am trying to describe, and I would thank you for your help.

Before you replace the boards you need to fix what caused your current problem, bad or absent gutters and down spouts. Can you tell us how close your siding comes to the grade? If you still have any of the warranty literature I'd get on the phone and ask the manufacturer to replace the boards.

The bottom of the boards are 14 inches from the grade, and the warranty is a few years over the 25 years.
I think the problem is caused from the splashing of rain water off the ground or just the water running down the wall itself.
We have painted that area about every third year, it is a very harsh climate there.

Well you got more than the warranty so it isn't too surprising that there is some failure. 14" should be fine, 24 " would be better. I'd replace the bottom 4-5 courses and let the owner in 2032 worry about the next repair.