I repainted my first plane but I don´t know how to change the daylight textures into night textures. Please help me!

If you are talking about repainting an aircraft that already has the night textures, than you need an aircraft repainting tutorial found on most websites pertaining to this area.

If you are talking about an aircraft that doesn't have default night textures, then adding them means you need the source files that created the original aircraft, which are held by the author of the aircraft.  But it would just be easier to get another of the same a/c that has the night textures already.