To remove fungus, the lens has to be completely disassembled down to
its individual glass elements. Even then, the cleaning can be
successful only if the fungus has not etched the surface of the glass,
or attacked the coating.

I've managed to do this with non-zoom, non-AF lenses. (The disassembly
of zoom and AF lenses is a lot more complicated.) Fortunately, Nikon
lenses (especially older ones) are modular, and the glass lens cells
can be removed from the front and rear of the barrel without
disturbing the diaphragm or focusing mechanisms.

Some of the lens cells are single elements, and some are made of glass
elements cemented together. The problem is that the fungus often gets
into the cemented joints. At least in the older lenses, the cement
used was Canadian balsam resin. The safest way to remove this is by
soaking in an acetone bath for several days. Then the glass elements
can be easily separated.

The actual cleaning of the fungus can be done with 99% pure isopropyl
alcohol (that leaves no residue). In stubborn cases, rub carefully
with cold cream first.

In reassembly, the cemented elements should be re-cemented, in order
to minimize internal reflections. I haven't been able to find a ready
source for the Canadian balsam, which in any case is difficult to
apply (it has to be heated). There's an artificial substitute that
cures by ultraviolet light, but once set, it can't be dissolved with
acetone. (So you only have one shot at reassembly.)

To sum up, cleaning fungus from lenses is a tricky and iffy
proposition. It might be more practical just to buy a new lens.