how do i change the dimension of a box or line in autodesk LT. i used the  linear dimension tool to reflect the length....previously i used to use autodesk inventor and changing dimension was simply by double clicking the dimension.

Modify properties, you can over ride the text.
 
If, that's the question(?).

say my length is 5cm and i want to change it to 8cm,is it possible ? im using autocad LT 2005

Select and scale them all use 1.6 scale factor.

Double clicking the dim in inventor will change the physical length of said line or part.
So in acad, to make a dim AND line measured go from 5cm to 8cm, simply use the stretch command, sweep select the grips of both the line and the dim (at same time) and stretch the objects to the desired length.
Said line will now measure 8cm, and associated dim will show 8cm.
Or just scale it, like Rman said. whatever your goals.