You can change your subscription with the Plan selector in the Lab Settings. If the number of active lab members or storage spaced used is above the plan you would like to select you'll have to remove members and delete enough space.
Select a new plan and click the Update Plan button to proceed with billing for the plan. Existing credits for the month will be saved and used to pro-rate any price changes.
Note: Once a free plan has been upgraded it can no longer be downgraded back to free.