This how-to comes to you because I just had to go through this process for my wife on her iPhone 7 when her phone started to display alerts that her iCloud storage was almost full.
By default, iCloud is used in iOS to backup your camera roll as you take images with the phone and it comes with just 5GB of storage for free with options to subscribe and add more storage to your account.
However, if you are already part of the Microsoft ecosystem then you can actually shift over to using OneDrive for syncing your camera roll images to the cloud. These days OneDrive also comes with just 5GB of cloud storage at no cost. However, if you are an Office 365 subscriber or maybe grandfathered with more storage prior to the reductions Microsoft made a couple of years ago then you likely have much more than just the free 5GB allotment of storage.
To get started just download the OneDrive app from the App Store on your iOS device and sign into your Microsoft Account via the OneDrive app after it is installed.
Once you are signed in just follow the screenshots in this gallery for steps on turning on camera roll sync with OneDrive and turning it off in iCloud.
Note: Make sure you allow the OneDrive app to sync your existing camera images from your phone to OneDrive first before shutting down the iCloud backup. I have bolded that warning in the gallery image to remind you. This is just to make sure that none of your existing images are lost in the transition.