Can I change my passcode on a stolen phone

This article (and links in it) tells you what you can do if you have a stolen device If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

You can remotely erase the device so if you have a weak passcode on the device the data won't be on it any more.

Erase a device in Find My iPhone on iCloud.com - https://support.apple.com/guide/icloud/erase-your-device-mmfc0ef36f/icloud - "If your iOS device, Apple Watch, or Mac is lost or stolen, you can erase it if you set up Find My iPhone on the device before it was lost."

- "After you erase a device, you can’t use Find My iPhone to locate the device or play a sound on it."

- "Activation Lock remains on to protect the device. Your Apple ID and password are required to reactivate the device."

No. The only way to change the screen lock passcode requires you to have the device in your hands.

No.