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.

This is a pretty difficult replacement, since it involves opening up both halves of the system. This is a video on how to fix a cracked hinge by replacing the midframe: https://www.youtube.com/watch?v=dVn_m_OI... I'd give it a quick watch before deciding whether or not to replace it. Assuming that you are a beginner to this sort of thing, the risks to damaging your system are pretty high. (I did this to fix a cracked hinge, and regret not being more careful, as my circle pad no longer works. But now I have an excuse to buy a New 3ds, so I suppose it's not all bad.) Most steps to the process are in this guide: Nintendo 3DS XL Touchscreen Replacement However, in order to re-attach the top screen, you will have to remove its outer shell, as shown in this guide: Nintendo 3DS XL Upper LCD Display Replacement From there, just follow the first video, taking care to not damage the wires. These can be slid through a slit in the casing by the hinge. The main things to watch out for are - Don't strip any screws. If...

In order to fix the hinge by replacing the center casing, a near-complete disassembly of the system is required. https://www.youtube.com/watch?v=dVn_m_OI... This video shows the complete process. If you are a beginner to the repair process, I would suggest finding a local electronics repair center to do it for you. But if you are confident, then most steps for this process can be found in the touchscreen repair guide here: Nintendo 3DS XL Touchscreen Replacement Make sure to get a good screwdriver and to be careful with the wire attachments.