I have a friend pursuing engineering in IET Lucknow, here are some tips for you.
There is almost no chance to get a seat in IET Lucknow according to your rank. If you will be lucky then you can get a seat. You can change branches after completing the first year.
You will require a good SGPA in both of your semester exams and should be getting under the top 10 then only you will get into the CS branch.