Since no harpers have responded, I will offer that the first thing you should do is to look for the maker's name. If it's not obvious, look for a label inside the sound box. Then see if the maker has a website, and send them an e-mail asking for help.

Plan B would be to locate the nearest harp teacher. If you are American, check the site for the American Harp Society.

Plan C would be to take it to a good hardware-store employee, point to it, and say, 'How do we tighten that?' However, I would go about the adjustment in a gingerly manner after that.