Changing 6 characters password to 4

Info...Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

• Change Passcode: Enter a new six-digit passcode. Or tap Passcode Options to switch to a four-digit numeric code, a custom numeric code, or a custom alphanumeric code.

Go to Settings > Face ID & Passcode. Tap on 'Change Passcode', provide your current passcode, then when you see prompt for new passcode you should see 'passcode options' above the on-screen keyboard.

You might not see option to choose '4 Digit Passcode' option if your device is uses configured by the company you work for using MDM/Profiles or if you have an Exchange account active.