Hmm the only proper way to sort this specific problem in my opinion is to take it to be re-trimmed at a professional auto trimmers, the downside is this costs anything from £50-£500 depending on the quality and time spent on the job.
The cheapest way for fastest results is a high quality black leather dye in the closest shade to factory black as possible (although any black will realistically do!). Use a nice consistent application pad such as a small foam pad and spread evenly over the rim. But make sure you give it a good 48 hours to dry properly - I would also recommend removing the steering wheel to make a really good job.
Last choice is just to give it a deep clean, Zymol do a good leather cleaner which smells good and reacts well to dirty and slightly worn leather, however for anything more than this see options above!!