In most states, no. If you live in a HealthCare.gov state, Open Enrollment for 2021 coverage ends at 3:00 am ET on December 16, 2020. You will not be able to change your 2021 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 plan for next year during Open Enrollment 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.