Why i cant upgrade my software in iPhone 5?

Do you have any error message? Please describe issue in more details.

Turn your device off and back on. If this doesn't help, sign out from your account and sign back in.

Also, you can try resetting your settings.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

https://support.apple.com/en-us/HT201559

Take a look at these articles:

Resolve iOS update and restore errors in iTunes - Apple Support

Get help with iOS update and restore errors - Apple Support

What is happening when you try? Have you looked at this link: Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support?

is it possible that you have the iPhone 4? -AJ

Are you trying to update over wifi or through iTunes on a computer?