how can i back up my phone if it is already locked

The following will cause a daily backup to run, while the iPhone is charging:

Follow these steps to set up iCloud Backup to automatically back up your device each day:

1. Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.
2. Connect your device to a power source.
3. Connect your device to a Wi-Fi network.
4. Make sure that your device's screen is locked.
5. If you get an alert says that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to buy more storage.

If the iPhone is Disabled:

If you mean to try to get a backup once your iPhone is disabled (such as after entering an incorrect passcode too many times), you can’t. The data is already gone. Backups have to happen before the loss, theft, drop, dunk, or disablement.