How reset just my passcode on my iPhone

This answer assumes that you know the passcode and get into the phone.

Settings > Face ID & Passcode > Change Passcode

However, if you are really telling us that you have forgotten the Passcode, or you are currently locked out of the phone, then you will have to erase / reset the phone and then set it up again with a new passcode.

You will lose the data on the phone unless you have a current backup to use to restore the data back to the phone after it has been set up again