There is an object on which I want to put the build modifier but inverse. Like the object is ‘un-building’ up (if there is such word) 
that also from the middle of the scene. Like at 40th frame. But the build modifier shows no object in 0th frame and starts building at 40 and finishes up. I want - there will be the object from 0 to 40th and from 40th, it will un-build up and disappear.
I tried to alter the keyframes in dopesheet, but didnt work. help please.
You can use a particle system with the explode modifier to ‘unbuild’ something.
See the attached blend file. Frames 1-50 use the build modifier to create the cube, then from frames 100-150 there is a particle system that the explode modifier uses to take it apart again. The particle lifetime is just 1 frame so the face just disappears.
Attachments
build.blend (181 KB)
Build would be so much more conveniently useful with a simple “percentage of object” value to keyframe.
But you can achieve pretty much any result by keyframing the existing values.
Here’s an example for a 80 frame “unbuild”:
Length: 80
At frame 1: Keyframe Start at -80
At frame 80: Keyframe Start at 80
(you probabably want to switch the animation curve to linear)
I’m never really able to wrap my poor head around the time-logistics, but it’s not that hard to trial-and-error a satisfactory result.
Save often!