From Settings > Account, in the "Subscription" block, click "Change plan" to move to a higher or lower plan.
Moving to a higher plan applies immediately: you get access to the new features and additional sites right away.
Moving to a lower plan takes effect at the end of your current billing period: you keep your current plan until then. If your new plan allows fewer sites than you currently have, only your oldest connected sites are kept within the new plan's limit; the others are automatically disconnected on the date the change takes effect. If your account has invited team members, they lose access if the new plan no longer supports team management.
A summary of what will be kept or lost is shown before you confirm the plan change.