Reset old phone

i don't exactly know what you want to remove from it but you can go into (on your old phone, if its an iPhone) settings, tap on the big banner thing on the top of the screen and then scroll to the bottom and i believe it should show some red "remove this device" thing, this'll remove it from your iCloud account, but if this isn't the right spot you can go to the same screen on your NEW iPhone if your new phone is an iPhone) and go to that big banner thing but instead of scrolling to the bottom, you can just tap on "iPhone ~whatever~" (so basically the icon that your old phone is) and then click remove device (on the old phone), this should appear after you tap on the device in this screen and once you do this it'll remove your old phone from your iCloud account.

if you want to factory reset your old phone (if its an iPhone), you go in settings (on the old phone), general, scroll down and tap on reset (it'll open more options), and you tap on "reset all content and settings" (or something like that, i'm doing this all off of memory so please bear with me if i'm a little wrong on wording, but i'm pretty sure this is what it says), this'll reset all your stuff on that phone and in fact after its done with this process, all your stuff on there will actually be gone and it'll literally take you to the welcome screen (also don't worry, its supposed to be on this screen, from here you can just power off the phone), i hope this helps and if you have any other questions, comments, or just anything in general, feel free to ask/say and i or someone else would love to help further