Can I use icloud to back up all of my iphone data?

Turn on iCloud Photo Library:

iCloud: iCloud Photo Library overview

Get help with your iCloud Photo Library - Apple Support

iCloud Photo Library FAQ - Apple Support

Can I use iCloud Photo Library to save space on my device?

iCloud Photo Library automatically keeps all your photos and videos in the original, high-resolution version. Follow these steps to choose how you store your photos and videos on your device:

• On your iPhone, iPad, or iPod touch, tap Settings > iCloud > Photos or Settings > Photos & Camera, then select a storage setting.
• On your Mac, click Photos > Preferences > iCloud, then select a storage setting.

If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them.

If you turn on Download Originals, iCloud Photo Library will keep your original, full-resolution photos and videos in iCloud and on your device. Download Originals is the default setting for iOS devices with the free 5 GB storage plan and for all Mac devices.

You can sync them to iCloud, but they won't be on iCloud *instead* of your phone,. They will be on both.