Sell your broken iPhone then add to that enough to purchase an iPhone 5. There are no upgrades or trade-ins with Apple. If your current phone is broken you may have it replaced with a refurbished one of the same model:
If your iPod Touch, iPhone, or iPad is Broken
Apple does not fix iDevices. Instead they exchange yours for a refurbished or new replacement depending upon the age of your device and refurbished inventories. On rare occasions when there are no longer refurbished units for your older model, they may replace it with the next newer model.
You may take your device to an Apple retailer for help or you may call Customer Service and arrange to send your device to Apple:
Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
To contact product and tech support: Contacting Apple for support and service - this includes international calling numbers.
There are third-party firms that do repairs on iDevices, and there are places where you can order parts to DIY if you feel up to the task. Start with Google to search for these.