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

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.

This would require pictures and more specific discussion or an exam to render a decision.Find a board certified plastic surgeon who performs hundreds
of rhinoplasties and rhinoplasty revisions each year. Then look at the
plastic surgeon's website before and after photo galleries to get a
sense of who can deliver the results. Kenneth Hughes, MD Los Angeles, CA

Nostril surgery is also called alar base resection or Weirs excisions. Depending on how these excisions are designed, they can remove nostril fullness, and/ or make the nostril opening smaller. I find that for patients who ask about this procedure, computer imaging can help make sure that doctor and patient have the same goals and expectations.

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.  

Nostril size and shape can be modified if subtle changes are desired,but careful planning is necessary. If there is fullness in certain views that you don't like, seek consultation with a rhinoplasty expert who can examine you and discuss your options. But don't be too eager- nostril reduction or shape changes often come with the possibility of conspicuous scars. Look at the surgeon's prior photos to get an idea of what these scars really look like, and also undergo digital photo morphing to more fully communicate your desires with your surgeon.