iCloud automatically backs up your iOS device information daily over Wi-Fi when your device is turned on, locked, and connected to a power source.
For more detailed information visit the Apple Support site - https://support.apple.com/en-us/HT203977
How to Back Up to iCloud Manually
- On your iOS 9 or iOS 8 device: Go to Settings > iCloud > Backup, then tap Back Up Now.
- On your iOS 7 device: Go to Settings > iCloud > Storage & Backup, then tap Back Up Now.