IIIT Guwahati offers only two courses currently in its undergraduate B.Tech program; ECE and CSE.
Currently, no cut-off CGPA exists above which you can apply for the branch change. Try to score as high as possible and try to be among the very top few of the branch. The higher your CGPA is, the better your chances are for a possible branch change.
It is suggested it would be better if you can aim for a CGPA of 9.2 and above.
But I would also suggest you not burden yourself with the grades and numbers too much and enjoy your studies as much as possible. Study well and strive hard for your success and definitely, you can do wonders at any branch.