reset

See the following Apple Support article for information on what to do to an old iPhone before selling, trading or giving away: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If I understand your very short post, you want to completely erase and reset your old iPhone so you can re-home it. Please see Apple's advice in

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support