Hi Rupesh,
You can do this by going to 'Edit | Edit Data | Unit Cell...' Click on the button marked 'Option...' and modify the transformation matrix.
To swap the b and c axes, for example, the matrix should read:
1 0 0
0 0 1
0 -1 0
(The minus sign is needed to maintain a right-handed coordinate system.)
Hope that helps,
Philip