Your iPhone automatically backs up to iCloud once a day, not every time something changes. It has to be connected to Wi-Fi, charging, and the screen has to be locked.
It's not possible to change this feature. You can backup manually, as often as you wish to.
To backup manually, go to Settings - your information at the top - iCloud - iCloud Backup. Written in blue, you'll find "Back Up Now". When you tap it, your iPhone will start backing up to iCloud via Wi-Fi.