I don't know how to and I am really confused.

Hi! First you need to be in a Tribe to do that.
If you mean the default map that plays in the Tribe House: While the Transfromice Tribe Guide might be somewhat outdated, it can give you an idea on where to go for that. Under the tab in the topic labeled "Managing" shows an old image showing the Tribe house map @code and the "change code" button.
Though the current way is to first click on the Cheese Shield button on the bottom left of the screen to pop up the Tribe Menu (if you're in a tribe). Next click on the Options/Gear button on the right then Click on "Define the code of your Tribe's house". Then insert the map code.

If you mean just changing the map to any other map while in the Tribe House room: Type "/np @#######" or "/np ##" in the room chat. Replace # with whatever the actual map number is. If you were to just type "/np", it would play whatever the current default tribe house map is. And if you were to type "/npp @####" or "/npp ##", the next map that plays after the timer runs out will be that map.

In addition to needing to be in a Tribe for these, you also need the proper tribe rank to do so. Any rank that's not Spiritual Chief can have any variety of powers depending on what the leaders have selected. Changing the Tribe house and loading maps in the tribe house room are two separate options.

If you have any other questions feel free to ask in the Questions & Answers topic in the TFM Discussions subforum: https://atelier801.com/topic?f=6&t=1535