If it is just the hinge that is broken (I'm assuming that's what you mean by 'broken in half'), then you will need to replace the center housing. It's a difficult process for a beginner, and can cause problems with the system if done incorrectly.

A video of the process:

https://www.youtube.com/watch?v=dVn_m_OI...

Most steps in the process are here: Nintendo 3DS XL Touchscreen Replacement

In order to reattach the top screen, remove the shell of the top screen as shown in this guide: Nintendo 3DS XL Upper LCD Display Replacement

From there, follow the video I posted before.

Take care to not strip any screws or break any grey flaps connecting ribbon wires to the motherboard. I only say this because those were the two main issues I have had while repairing my system. I haven't been able to resolve the latter, and now my C-Pad is unresponsive, so learn from my mistakes.

If this feels above your current skill level, then I suggest finding an electronics repair center to fix it for you.

Good luck.