This only applies if your account has the Plan module.
Go to the ‘User’ tab and select ‘Subscriptions’ for the user whose subscription you want to manage.
Select ‘Add Subscription’ and select the plan you want to add. Optionally you can set this when the plan expires.
Select ‘Save’ to save the changes. You can also unsubscribe a user from a plan by clicking on the ‘Unsubscribe’ button and clicking ‘Save’.