After completing Diploma in electronics, can I change my branch in BE? What is the process of the same?

No, you cannot change your branch. You must do BE in the same branch which you opted in Diploma.

You should change your branch firstly. And No you cannot change it. You will directly get admission in 2nd year of B.Tech with same branch. I think you will do it better if you would have same branch. All the best.