How do I get more storage so I can update

You are unable to purchase extra storage for your iPhone device, however, you are able to purchase an iCloud+ subscription, which is Apple's cloud storage service.

If there's not enough space to update

If there isn't enough space to download and install an update, the software on your device will try to make space. It will remove only app data that it can download again and doesn't remove or relocate any of your data. If your device still doesn't have enough space, you can use your computer to update your device.

If you can't connect your device to your computer, you can make room for the update by removing content and apps on your device that you don't use. Go to Settings > General > [Device name] Storage.

Find out more about the space your apps and content use.

Source and More Information - If your iPhone, iPad or iPod touch won’t update – Apple Support (UK)

You can release more space to your phone by deleting apps you dont need or deleting pictures. You can also send pictures to your iCloud so they would not take space from your phone. If you need more iCloud space you need to buy it from apple.