APIs can be accessed via Swagger UI, which is available at <You JIRA address>/rest/testy/3/rest-browser/. For example, the address to our public demo Jira instance is https://demo-structure.almworks.com , so the URL for Swagger UI is https://demo-structure.almworks.com/rest/testy/3/rest-browser/
Before trying to access Swagger UI, you must log in to your Jira instance first.