Aqil - follow the step below and your part should be able to rotate in the assembly.
Rotate the part by choosing the Rotate Component from the Move Component drop down menu.
You can also rotate the part by right clicking the mouse and choosing Move with Triad.
If a (f) appears beside the part name in the history tree it will not rotate because it is Fixed. Right mouse click and choose Float to allow the part to be moved or rotated.
If a part still will not move in an assembly make sure there are no mates that are stopping the part from moving.