In most states, no. If you live in a HealthCare.gov state, Open Enrollment for 2020 coverage ends at 3:00 am ET on December 18, 2019. You will not be able to change your 2020 plan during the year unless you have a life change that makes you eligible for a special enrollment period.
It is advisable to review your plan options and actively select your 2020 coverage before 3:00 am ET on December 18 to ensure you have made the choice that is best for you.
State-run Marketplaces can set different dates for Open Enrollment and several have done so. Check with your state Marketplace for more information.