Can't find API documentation on Order Editing APIs for post-purchase offering and upsell

[chenster]

I'm still a rookie Shopify app developer, so bear with me.

On Everything We Announced at Shopify Unite 2019, also this post, it mentioned a new Order Editing APIs that can do post-purchase editing and even upsell. However, for the life of me, I could not find any documentation on Shopify about those exciting changes. Please someone points me where to locate the information to build an app from the new API? 

Also, I'm aware those are still in developer preview, so how can one enable the preview in order to gain access to the new upcoming apis?

[chenster]

Just to share that I did find order editing API, but it is Admin API, used by merchants, not by end-users.