The iOS/iPhone/iPad app for Warmshowers is in the Apple app store.
Please upgrade to the newest version. If you have any trouble at all, please uninstall and then reinstall the app.
Code for the app is at https://github.com/warmshowers/warmshowers-iphone-app and issues are handled in the issue queue by volunteer maintainer Chris Meyer.
The units option is located under the iPhone Settings WS page.