After the completion of your diploma in civil engineering you have to give the lateral entry exams for taking admission in B.Tech course. Generally, it happens that institutes have their own guidelines and they don't allow the students to change their trade. But there is a possibility that if your past academics and diploma grades are good, they alllow you to change the trade. Again it's institute specific as every institute has its own set of rules to follow. Best would be that you enquire about this with the institute administration before taking admission that whether they provide this facility to students, if provided what are the conditions which needs to be fulfilled.