You said that the dying leaf was an outer one, so it's probably part of the normal life cycle. I would just let it alone. It will fall off when it's ready.
If you really hate looking at it, or, more importantly, it's got dark brown wet spots (a sign of disease), than cut it off with a sterile blade, as close to the leaf base as you can get without injuring the other leaves. The dying leaf should be soft enough for you to gently pull it downwards to open up enough space for you to do the cut.
The cut itself can be made with a fresh single-edged razor blade (available at hardware stores), a sterilized hobby-type knife (like an Xacto), small pair of sterilized scissors (like a cuticle scissors) or anything similar.
But, as I said, I'd leave it alone.