Yes, you can always downgrade to a cheaper plan by selecting the desired plan in the billing tab.
The downgrade will take effect in the next billing cycle.
When downgrading between paid plans, the main difference is the number of products you can discount per month. However, some features are only available on specific plans, and downgrading may affect your active campaigns.
When downgrading to the Free plan, you'll be prompted to deactivate any campaigns exceeding the limit (only 1 is allowed) or those using paid features.
When downgrading from the Ultimate plan, you'll be prompted to deactivate any campaigns using the "Apply sale prices to markets with fixed prices" option.