Yes, there is a provision for branch change at IIT Mandi. The exact number of students allowed to change their branch depends on the number of available seats in the particular branch. But as a trend, almost 10 out of 40 (close to 25%) of students of a particular branch can change their branch in a given academic year. Here are a few guidelines issued by the institute regarding the branch change.
In very rare and exceptional cases, the senate may award branch change to a student before the fourth semester if there exists any genuine and compelling reason for the same. Hope this helped.