Nintendo has just released Nintendo 3DS firmware update 5.1.0-11U, as they hope to improve the performance of their handheld system's core applications.
According to the official Nintendo listing, firmware update 5.1.0-11U resolves issues whereby some users were unable to access System Settings, the Nintendo eShop or Game Notes. This issue arose when Nintendo release update 5.0.0-11U last month and it occurred when consumers attempted to download the firmware update.
This update should prevent this problem from happening in the future, but if you're still suffering from a similar issue, Nintendo recommend going through their troubleshooting section.
Aside from that, update 5.1.0-11U adds nothing else to the Nintendo 3DS' functionality or stability. It can now be downloaded via wireless internet connection.