Your API definitions are organized into a set of JSON files. When the number of files is small, it is easy to find the appropriate API file. You can simply find it in the File tree or use the Open option on the Home Ribbon.
But, eventually, both the number of APIs and API files may become less manageable. This is where the Find API File tool is useful.
From here, you can use a combination of filters to find the APIs that you have previously defined.
Once you find your API, double click it and that API will be shown within the API Designer.