Diploma

Aptech, Jetking, NIIT are the best institute for Diploma in networking.

You can go to the institute from where you want to do Diploma and apply there for admission. Some institute also take entrance exam for admission in Diploma.

You can get the admission procedure by given link:
https://www.shiksha.com/college/vaishno-devi-institute-of-studies-dum-dum-park-Kolkata-36590/courses.

Please check for your eligibility to pursue this course from the criteria mentioned below:
P. G. Diploma in Clinical Nutrition & Dietetics: Eligibility:
Admission is offered to applicants who have successfully completed a Bachelors degree with a specialisation in the related field accredited by ICAR. Regards.

Check out the link below for information and for detailed information, download the brochure from the link below:
https://www.shiksha.com/engineering/course/diploma-in-computer-engineering-government-polytechnic-pune-shivaji-nagar-275292

To get the best colleges for B.E computer engineering through lateral entry, I would suggest you to apply for state LEET (lateral entry test) examination, score good in the same and get the best colleges.

Yes, it is definitely possible. Apply for it and the usual procedure will be followed from LPUNEST, then fee submission followed by in campus formal documentation. Still, you have given two years to CSE, take your decision wisely before switching the branch.