There should be a link to it from the My Account dashboard. See screenshot below
After you’re on the main “My Account” page, click on the menu item for “Store Credit & Coupons”
How do I use my store credit?
You can use the store credit on any subscription, workshop, physical products, or anything else sold through the Yoga Union Online store. Once you’ve added an item to your shopping cart, a button showing your store credit remaining will be visible in the cart. Click on it to activate it.