For HealthCare.gov, the increased premium tax credits should be visible by April 1, 2021.  Current marketplace enrollees can log in and update their applications to get new results starting on that date.  You will need to reselect your current plan in order for the changes to take effect to reduce your premiums for the remainder of the year.  State-run marketplaces are also working to implement new changes as soon as possible. 

Current enrollees can also decide if they want to change to a new plan for the rest of this year.  With increased premium tax credits, options you may have considered during open enrollment (for example, with lower deductibles) may now be affordable.  HealthCare.gov is open for new signups and plan changes during a special COVID enrollment period scheduled to run through August 15, 2021.  All state-run marketplaces also have a special COVID enrollment period this year.  Some are only open to uninsured residents at this time, though that may change in light of new subsidies.  Check with your state marketplace for more information.