It's simple, sign in to your mypura.com account and go to Account. Click on My Subscriptions and select Manage Subscriptions. Find the product you would like to swap, and click Manage this subscription. At the bottom of the page you will see the Swap this for a different product button. Click this and from there you will be able to select the product you would like to swap to.
Select the size, quantity and delivery frequency for the new product and finalise the swap by clicking Swap Product.