If you are using online features on your Nintendo 3DS or Wii U system, you most likely have a Nintendo Network account. If you do it is strongly suggested you change your password now.
This month around 450 Nintendo Network accounts got hacked, and the full details of these accounts were posted online in a list. This means if anyone manages to find it that person can easily get into your account.
If you want to check if your account is affected, click here and use Ctrl+F on PC, cmd+F on Mac or the find feature on your mobile web browser to enter your Nintendo Network ID and see if you are in the list.
If you are in the list, it means your account has been compromised and you need to change your password. If you aren’t in the list, we still recommend you change your password.
You can do so via the System Settings on the 3DS and Wii U systems.
Source: Gaming Reinvented