Follow these steps
Step 1:
Click on your Profile Name from the top-right hand side.
Step 2:
Select Profile & Settings.
Step 3:
From the left-hand side menu, click on Subscription.
Step 4:
Click on Edit from the top-right hand side.
Step 5:
Locate the Core Subscription Details tab and click on Upgrade Plan.
Step 6:
From the dropdown, select the respective subscription plan you want to downgrade to and select the required user licences.
Step 7:
Once the details are set, click on Update from the bottom-right hand side.
Step 8:
Review the updated subscription amount and click on Confirm to complete the downgrade.