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.

Please note that when downgrading between paid plans, only the number of products is affected. However, downgrading to the Free plan also limits available features. So when downgrading, you'll be prompted to deactivate any extra campaigns (only 1 is allowed) or those using paid features.