If there isn't enough space for an update on your device, try this.
Updating to the latest version of iOS or iPadOS software provides the latest features, security updates, and bug fixes. If a message says that there isn't enough space to download and install an update, manage your space with recommendations or connect to your computer to install the update.

To make space on your device, try the recommendations. You can also delete or offload apps and content. You don't need to purchase more iCloud storage. What's the difference between device storage and iCloud storage?
If you see Other or System Data in the graph that shows your device storage, you don't need to take action to change them. Instead, delete apps and content that you don't need, and use the recommendations.
If an alert appears during an update, learn how to resolve an error or alert.