Yes, a paid subscription goes with the free 7-day trial period at the beginning of the subscription where you're given service for free until the first bill is issued. You won't be charged until your trial ends and can cancel the subscription at any time.
NB: the free 7-day trial period does not apply to a Lifetime Subscription. Please be sure to test a regular subscription before considering purchasing a lifetime option.
You can do it either from your Account Settings (Account > Settings > Change Subscription > Want to cancel your subscription? Click here.) or using the special link provided in the order confirmation email.
Quantites and/or features of the underlying plan can be changed by buying a new subscription. The underlying subscription should be cancelled to avoid double billing.
FlexiHub is licensed according to concurrently connected devices. For example, you can install it on 100 machines but pay for a 10-connection subscription if you anticipate only 10 devices being connected at the same time.