how to change the icloud backup frequency in my Iphone 11?

iCloud backups, if enabled, will run automatically whenever all the following conditions are met: You can't change this on iPhone.

1. The device is powered on,
2. The device is connected to the Internet via WiFi,
3. The device is connected to a power source,
4. The device has the screen locked (i.e. turned off)
5. The device has not been backed up successfully in last 24 hours.

If you can't back up to iCloud ...

It's not automatic so you need the tell it to backup.