I have created a Qt app for the Nokia Ovi store but I know want to update the font int the QML file, but just changing it in the designer doesn't change it on the symbian device. I have checked google but nothing I can find looks right. If it helps the font I want to use is Kristen ITC but it might change.
If you want to use a custom font, you'll need to deploy that font along with your application. This blog article should help you out, though I don't use Symbian so perhaps another answer can go into more detail.
Once font is available on the device, you'll need to add it to your application using QFontDatabase::addApplicationFont.
You'll also want to look into licensing whichever custom fonts you deploy with your app. Kristen ITC, for example, is included with Microsoft products, but licensed by the Monotype Corporation.