Update IOS from ios 7.0.4 to ios 12.4.5 fail.

Error 3503 is associated with multiple issues and one of the common ones is that iTunes on your Windows PC needs to be updated to the latest version. Update your iTunes to the latest version then try updating your iPhone again.

Axel F.

Let's try the basics on your iPhone -

• Turn VPN OFF is it on in settings.
• Try another WiFi network
• Toggle Airplane mode on and off
• Update your Date & Time to Set Automatically or toggle set automatically off and on
• For iPhones and iPads with cellular service, try toggling off WiFi and use cellular data instead (this may incur additional mobile data charges as updates tend to be large files)
• Reset your network settings - Open Settings ➔ General ➔ Reset ➔ Reset Network Settings
• Restart or force restart your device ➔ How to Force ReStart Your iPhone.
• Remove any beta profiles, if applicable

Axel F.

You have not updated the software in a really long time. iOS 7.0.4 is from late 2013. You cannot perform a direct update (using only your iPhone) anymore, you need to update it using iTunes on a computer.

Get help with iOS updates

https://support.apple.com/en-us/HT209571

Get help with over-the-air iOS updates

https://support.apple.com/en-us/HT201435

Get help with iOS update and restore errors

https://support.apple.com/en-us/HT204770

I understand that you have updated your iTunes. Thats a good thing.

Next step is to Check security software on your computer

Security software like anti-virus application or firewall installed on your computer may not let you access iTunes properly or stop iTunes to connect with the Apple servers. Either change the settings of the third-party software installed on your computer or disable the software until the update is complete.

Axel F.