download

Please read the message carefully. It is telling you that your iPhone does not have enough available storage space for the update. You will need to first remove the excessive amount to try the update. Typically, you would need, at least, 5 GBs of free space.

You also need to connect your phone to a Wi-Fi network with Internet access.

Get information about your iPhone:  

View overall storage availability and storage used per app:

Get information about your iPhone - Apple Support

Update iPhone manually: 

To turn off automatic updates, go to Settings > General > Software Update > Automatic Updates.

Update using your computer:  

Update iOS on iPhone - Apple Support

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Hope this is helpful. 

Have a great day !

Update iOS software on iPhone - Apple Support

Please read the message at the bottom of your screenshot

Also: Connect to a WiFi network.

Also: Charge the battery and preferably leave it connected to the charge circuit during the update process.