Update

To delete items in iPhone Storage as suggested in your screen shot, open Settings > General > iPhone Storage. Here, you will see how much storage your various apps are using. Delete any apps you are nor using by tapping it then selecting Delete App.

Another good thing to do is to open the Photos app. It is easy to accumulate hundreds, if not thousands, of photos you no longer need. See what you can delete to open up more storage for the update.

As the bottom of the screenshot shows, you need to clear more disk space on the phone or attach it to a computer Mac computer to update the IOS.

See if anything here helps. 

Disk space - How to check the storage on your iPhone, iPad, and iPod touch.    *

 Disk Space – Free Up    -

 Disk Space – Free Up (2).  -

 Disk Space – Free Up (3).  -

 Disk Space - Free Up (4)    -    

 Disk Space - Free Up (5)   -  

ASC YouTube for how to free up space on your iPhone, iPad, or iPod touch  

If you want to use a computer.

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *