This question was posed to me, and I decided it deserved a thorough (and expanded) answer.
1. If we focus on the nose itself, nasal passages can be physically obstructed by something like a deviated septum or enlarged turbinates. In those cases surgery is necessary. If congestion is the issue, seek the help of an allergist or try over the counter nasal medicines.
2. In some situations, night-time breathing can be much improved by using a device like Mute (https://goo.gl/UuBVbn) (and/or mouth taping). Obviously if there are underlying problems contributing to difficulty nasal breathing, these will not help, and may even harm.
3. In general, the more you nasal breathe, the easier it becomes. An unused nasal passage will have increased congestion, creating a vicious cycle.
A great technique to help improve and train nasal breathing is the Buteyko method. Patrick McKeown has a website, books, and YouTube channel - all of which are great! (https://buteykoclinic.com/breathing-exercises/)
4. Finally, look past the nose. All of the following can contribute to difficulty nasal breathing:
Enlarged tonsils and/or adenoids
Weak or tethered tongue
Poor tongue posture (tongue not resting against the roof of your mouth)
Other problems of the mouth and throat tissues
Jaws that are too small (think crowded teeth/narrow arches/high palate). This results in a smaller airway and less space for your tongue.
Ultimately, you need an airway that is capable of easily delivering an optimal amount of oxygen from your nose to your lungs. There are many, many reasons why that may not be happening.
Your airway team may consist of ENTs, sleep physicians, allergists, airway focused dentists, myofunctional therapists, and sleep hygiene experts. Sometimes the reasons for poor breathing or sleep are simple, but more often than not they are confusing and complicated.
For many adults, treating symptoms over root causes may be the most feasible or only option. But you also have the ability (and hopefully desire) to help prevent similar issues for the children in your life. Don't let anyone tell you that breathing issues in children are minor - without attention to root causes, their issues will only get worse. With early intervention, children can be on a greater path to health.