I can’t change my iPhone 10 password

Why can't you change the password? What happens when you try? Error message(s)? More information would help.

Or do you need to change the phone's passcode?

If you know the current passcode.

Passcode - Change and Help    *

If you forgot the passcode, see the Apple Support article(s) below. 

Passcode - If you forgot the passcode for your iPhone or your device is disabled.    *