If you need to test a website on an iOS device, you can use the iOS simulator to test without having to build or run an app (the usual way the simulator is opened).
You will need XCode already installed, then either right-click on the XCode icon and choose 'Open Developer Tool' and then 'Simulator', or from within XCode, click on 'XCode' in the menu then 'Open Developer Tool' and 'Simulator'.
If you need to change the device the simulator is using, once the simulator is running then click 'Hardware' in the menu and then pick the device you need from the 'Device' dropdown.