Desktop: in the documented X2View interface, choose Configuration → HMI Editor. HMIEditor is a subeditor of X2View. Its images and controls are saved in the project folder and uploaded with the project to the device.
Browser: the WEB HMI editor manual gives http://device-IP/editor.html as its entry point. It states that the custom properties opened by double-clicking an element correspond to those in the built-in HMIEditor. This is not a claim of complete feature parity; custom-library import, for example, follows different procedures.