To deactivate your profile, click on your profile picture (top-right) » 'Edit profile' » 'Account' » 'Deactivate' » 'Deactivate'.
Please keep in mind that FREE deactivated profiles are automatically deleted after 12 months. Automatically deleted profiles can't be brought back to life, so all of your saved messages, friends, pictures, and other information will also be gone forever. Deactivated PLUS profiles are not deleted.