How can I fix it if it broken in half

"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."