Generally you can cancel a dental plan at anytime, by not paying, if it’s a stand-alone dental plan. If you have a plan that includes dental, you can switch it during open enrollment, but can’t drop the dental part of the plan without dropping the whole plan. According to HealthCare.Gov:
If you have a separate, stand-alone dental plan, you can cancel any time during the year by not making payments on the dental plan premium. This will cause your dental coverage to end. As long as you continue to pay your health plan premium, you’ll stay enrolled in your health plan.
Important: Don’t cancel your dental plan on HealthCare.gov if you want to keep your health plan. Selecting “Remove” in My Plans and Programs under your dental plan will cancel both your dental and health plans. Instead, you can purchase the dental plans separately "off" the exchange, directly with a carrier such as Ameritas Dental.
If you have a health plan that includes dental benefits and want to modify that plan, you can change to another health plan that doesn’t include dental benefits any time during Open Enrollment. The 2019 Open Enrollment period starts November 1, 2018 and ends December 15, 2018.