My reminder app is not working

First, you will need to explain what you mean by "not working".

I'm sorry but that doesn't make much sense in English.

If you want to change the default list, go to Settings>Reminders>Default List and chose whichever one you want to use as the default.

When you add a reminder, you can also select which list you want to add it to, either by first navigating to that list or by selecting the list below the details section in the New Reminder pane.