Update, July 2022: Plan downgrades are now automatically triggered after 24 hours if your active client is within the lower plan limit.
You can only downgrade your billing plan onto a lower plan when the total active count of your clients does not exceed that plan's limits.
For example, if you're currently on the 100 Client Plan, but then reduce your active clients down to 50 or below (by moving them to inactive), then you would be eligible to downgrade onto the 50 Client Plan as you are within that plan's active client limitations.
To downgrade your plan manually (without waiting 24 hours for the automatic plan downgrade to happen), follow these steps:
Head to the 'Plan & Billing' area, found within your settings menu.
Select the 'Downgrade Plan' button. Note, this button will only appear if you are eligible to downgrade onto a lower plan.