Hi,

Hope you are doing well.

You already have a diploma. I guess it would be easier for you to enter engg colleges for Civil related courses. Btech is a 4 year Under graduate course in the Field of Engineering. For Diploma Students, it is a 3 year course in which the diploma students will directly go to second year of Btech. So No, you can't pursue CSE steam in Btech after pursuing Civil engineering in diploma course. To pursue CSE in Btech you must have done the same Branch  thst is CSE  in Diploma.

But no one cant stop you from learning coding and programming even mechanical and civil engineers work in ITs For ecample Sundar Pichai (CEO of Google) after learning coding by themself from YouTube, YouTube and other coding learning channels are  actually has very good video content which is far better than the degree courses and you actually get to learn real stuff.

Hope this helps.

Good Luck!