There are 2 ways available to test the upsell funnel in Thank you page.
Prerequisite: Already have an order (in Shopify Orders) that contains the product which is the trigger product of your funnel.
Go to Order Details Page
Do a test purchase of the trigger product
Step 1: Create a 100% Off discount coupon and use it for that test purchase.
Step 2: Don't forget to remove the 100% Off discount coupon after the test.
RECOMMENDATION: Use a random combination of alphabets to create the coupon. Which helps to make it difficult to predict for others.