How to backup a locked iPhone without the passcode?

To back up an iPhone without knowing its passcode, unfortunately, isn't possible. The passcode is a critical security measure, and without it, you can't access the device to perform a backup. If you've forgotten the passcode, the only option is to erase the iPhone, which will delete all data, including any unsaved information. Before doing this, check if you have any previous backups in iCloud or on a computer. If you had iCloud backups enabled, your data might already be saved up to the last backup date.

If you have iCloud backups enabled and you have not restarted/locked your device, a new backup may occur when the device is plugged in and locked overnight.