If you were enrolled in Medicare Part A and B automatically when you turned age 65, there will be a card in your packet that you can send back as instructed on the card. This will disenroll your from Part B Medicare.
If you signed up for Medicare through Social Security, or if you have had part B for some time and now want to drop it, contact Social Security to drop Medicare Part B.
It depends on your reason for wanting to drop Medicare Part B. If you are going back to work full time and your employer will put you on their group health plan you may want to consider dropping Part B of your Medicare coverage but I don't recommend you do that. You are only given one Open Enrollment period to get a Medicare Supplement without having to go through medical underwriting after you activate your Part B coverage. Even if you drop your Part B coverage and later on activate it again you will not be given another Open Enrollment Period. If you have any serious health conditions before you reactivate your Part B coverage you may not qualify for a Medicare Supplement. My advice is to keep your Part B coverage even if you are going to be eligible for health insurance through an employer.
If you signed up for Medicare through Social Security, or if you have had part B for some time and now want to drop it, contact Social Security to drop Medicare Part B.