can computer science graduate can enter in to mechanical engineering
You can switch the branch after a year. There's an option called "Change of branch" wherein you'll have to meet the prescribed criteria of GPAs and apply for the same. It'd vary from college to college. Do check the notice boards and consult your HOD about it.
Once you opt for seat and admissions process is over then you simply cannot switch between streams.
Hence, its better to be precise of where you are interested and choose seats accordingly to avoid any hassless later.
Only in some special cases, you are allowed to switch if some seats remain vacant even after 1st year if engineering or some students leave the college due to unfortunate reasons.