You could not have enough space on your device.
Read this support guide:
Get help with over-the-air iOS updates - Apple Support
If you can't update wirelessly, you need to connect your iPhone to a computer running iTunes latest version. Read also this guide:
Update your iPhone, iPad, or iPod touch - Apple Support
If you don't have a computer, borrow one from your friends, or go to your most convenient Apple Store or Authorized service provider. Apple - Find Locations - Location Selector
Regards
Giulio