I cant remember how to change passcode on an Iphone

Settings>Face ID & Passcode...

But if you can't remember the existing passcode, you won't be able to change it. You can only remove it, using these instructions:

Keeping in mind that if you don't have a backup all data will be lost on that device (except those that you eventually synced to iCloud), pay attention to the specific gestures that you need to perform while connecting your iPhone to a computer (Windows 10 PC running the latest iTunes version) or Mac (which can have iTunes or Finder, depending on the macOS installed on it), and follow these instructions to re-gain access to your device:--> If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

or watch this support video about the same matter:

https://www.youtube.com/watch?v=7s8-zLG9ySQ

Regards

Giulio