Unable to update a iphone

In addtion to what kb1951 noted,

"If you need more space for an update" click here --> https://support.apple.com/HT203097

Things to try (read the link to get more details):

Plug your device into your computer to update using your computer.

Delete content from your device and try to update again.

Manage the storage on your device with recommendations. Settings > General > [device] Storage > Scroll to the Recommendations

There are related tips in this document:

If your iPhone, iPad or iPod touch won’t update - Apple Support - "You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons."

https://support.apple.com/HT201435

1. you need to free up space. 2. You need to be on WiFi. 3. You need at least 50% battery or be plugged into power.