I want to update my software to iOS 14.0

It is not possible to update to iOS 14 on any Apple device. To update to the most recent compatible version:

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Please see reply above.

Please read the replies above.

What is the existing version of iOS on your device?

If it is iOS 13 or higher, it can be updated to iOS 15.5.