If possible, try to get CPT and save the OPT quota for your future.
After you accept the offer, you will get two copies of the offer letter from the company.
1a. Go to the Gradute office to get a sabbatical form for intern (CPT).
1b. Go to Office of International Student Program to change your I-20.(It
may take one or two weeks.)
2. Sign the sabbatical form by yourself, by your advisor and Option representative
3. Get your advisor's support letter (saying that this intern is a helpful training for your study)
4. Give one copy of the offer letter and your advisor's support letter and the sabbatical form to Graduate office.
Remember to keep a photo copy of each document by yourself.
This is roughly what I did last year. You can first go to Graduate Dean's office and ISP to ask if there is anything changed.