Rhinoplasty done several years ago and now I would like to change nostrils only. What is the best way to do this?

HiYou can narrow nostrils fairly easily but you cannot easily change the shape or nature or form or position of the nostrils.  The reason is nostrils are made up only of skin and there is only so much you can do to skin.  The rest of the nose is made up of skin and cartilage or skin and bone and by reshaping the cartilage &/or bone the skin take on the new shape and redrapes over the underlying structure.  The nostrils not so much.  There are some limited things that can be done but many procedures on the nostrils leave unsightly scars with very little improvement.  I would have to see you to determine what the issue is but just know that there are limits.  

Without seeing your photos at least, it is impossible to assess your case accurately. However, generally speaking -- it is possible to make nostrils smaller-- but if your skin is thick/and oily -- then your results will potentially be compromised  by it because final result in rhinoplasty surgery especially revision rhinoplasty is always dependent upon skin quality.

There are a variety of ways to alter nostril shape. Alar base excisions are commonly used for correcting alar flaring, diminishing nostril size or both. Additionally, the pattern of excision can be modified to narrow the width and reshape the alae. In some cases, these procedures can be performed without requiring surgery to any other part of your nose. You should consult with a board certified plastic surgeon to discuss your concerns and see what options you have.