Definitely use a heat gun to open the back, or else you run the risk of breaking the back, also be carful when taking it off near the home button not to damage the touch ID flex cable. Make sure to also replace the screen with the LCD/Assembly not just touch screen
+
+
Edit: Didn't realise the back was broken. If you want to you can take the back off now without a heat gun if you're going to replace it. Be carful still of the flex
Definitely use a heat gun to open the back, or else you run the risk of breaking the back, also be carful when taking it off near the home button not to damage the touch ID flex cable. Make sure to also replace the screen with the LCD/Assembly not just touch screen