how can change dimension of assembly part , I tried but not possible

on my point of view through my experience I would like to suggest you change your assembly dimensions get back to the part body change the dimension of the body it directly changes your assembly

This icon mean that your part is taken from another assymebly with multibody parts.
If you want to change its dimantions you need the full build tree of said item.
Go back to where you made the part and update it there.
For future better not use this technic