Since the 2019 Open Enrollment Period is over, you can now enroll in or change a Health Insurance Marketplace plan only if you have a life event that qualifies you for a Special Enrollment Period.

Changes in household
You may qualify for a Special Enrollment Period if you or anyone in your household in the past 60 days:
-Got married. Pick a plan by the last day of the month and your coverage can start the first day of the next month.
 
-Had a baby, adopted a child, or placed a child for foster care. Your coverage can start the day of the event — even if you enroll in the plan up to 60 days afterward.
 
-Got divorced or legally separated and lost health insurance. Note: Divorce or legal separation without losing coverage doesn’t qualify you for a Special Enrollment Period.
 
-Died. You’ll be eligible for a Special Enrollment Period if someone on your Marketplace plan dies and as a result you’re no longer eligible for your current health plan.

https://www.healthcare.gov/coverage-outside-open-enrollment/special-enrollment-period/