The accordion segment that's expanded in Designer Studio when the section is saved is the one that's expanded by default. So you should just need to open the section, expand the desired accordion segment by clicking the icon to the left of the accordion row header, then re-save the section.
Below, find some illustrative screenshots from the UI Gallery Accordion example.
1) Default General row expanded:
2) Open the section and click the circle icon to the left of the desired header:
3) Save the section with the new header expanded in Designer Studio:
4) The new header is now expanded by default at run time: