Uneven Nostril Height After Septorhinoplasty

This is a difficult problem to fix.  There are ways to make the nostril more symmetric but it seems you are referring to the nostril rim or ala more than the nostril itself.  A small cartilage rim graft on the higher side can bring the rim down and provide more symmetry.  A thorough evaluation by a rhinoplasty expert would be essential.

Although some asymmetry may still remain using an alar rim graft is the easiest and least intrusive option 

My advice would be to leeave it alone. Some asymmetry is normal. To change the position of the rim is very difficult to achieve and will leave a scar along the base of the nostril.