To change your pricing plan you will need to go in contact with the Rocketspark support team and let them know which plan you are wanting to move to. They will then make this change for you or let you know that you are not able to due to reaching product limits of that new plan.
To get in contact to them, you can email them directly at support@rocketspark.com
NOTE: Before decreasing your product limit, please ensure that the current amount of products you have in your shop is below the limit you are changing to. For example, if you are changing to the "up to 400 products plan" please make sure you have 400 or fewer products. If you are changing from an E-commerce plan to the Website Builder plan, you will need to ensure that you have 10 or fewer products before changing your plan.
To delete products from you store, please follow this guide here: How do I delete a product in my shop?
See www.rocketspark.com/pricing to see what is included in each plan.