Change the passcode on iPhone

You can't just change the passcode of a device, that would defeat the purpose of security if you could change it without accessing the device in the first place. You will find: https://support.apple.com/en-us/HT204306 is going to be helpful in resolving a forgotten passcode issue. It does involve erasing your device. If you don't have an iTunes or iCloud backup, you will experience data loss.