OT: And don't forget to have your battery charged up! IIRC, typically there is a threshold and below that the update won't install.
Edit: Just saw
this link in another thread. iOS will let you know how much to delete if it fails. From the link:
When updating wirelessly, your iOS device needs enough available space to download and install the update. If "Download and Install" is grayed out or you see the message "This update requires at least x.x GB of available storage", use one of the following options to update:
◾Use iTunes to download and install the update instead
or
◾Remove content you no longer need from your device, including:
<snip>