Go to "System Settings" (the gear icon, usually bottom right). Look for the Kanji: 言語 (Language). If you are lucky, the AVIC-RZ03 may have a hidden English language pack. On most JDM units, this is greyed out, but some firmware updates allowed it. If you see "English" listed, select it immediately – you are saved.
Since an official English manual doesn't exist, most users rely on visual identification of the Japanese menus. You can use translation apps like Google Translate with its camera feature to read the screen in real-time. Common Japanese Menu Items Translation システム (Shisutemu) (Mokutekichi) Destination (Genzaichi) Current Location Community Workarounds Partial Language Change: pioneer carrozzeria avic-rz03 english manual