If you are looking for a "sneak peek" or summary without buying, search for the "API 6A 21st Edition Addendum" or "Errata" on the API website. These documents (which are free) often list the specific technical changes from the 20th edition.
You can download the API Specification 6A, 21st Edition PDF from the API website or other online sources. api specification 6a 21st edition pdf link