How to set a new passcode on iPhone?

Assuming that you know the current Passcode......

Settings > Face ID & Passcode > Change Passcode

Now, if the reason that you want to change the Passcode is that you have forgotten the existing Passcode and cannot get into the Settings at all.....things are going to be a lot more involved.

Post back for more instructions in that case.

Here are the instructions from Apple, which are the same that I provided to you.

Set a passcode on iPhone - Apple Support

If you are saying that Face ID & Passcode do not appear under Settings, I cannot duplicate that. Face ID & Passcode appear on both of the two iPhones that we have here, just as the instructions illustrate.

Does another user have access to your iPhone? They may have set up Restrictions that need to be changed.