UV light will kill the fungus, so all you need to do is let the lens be exposed to sunlight for a few days. Be sure to remove any UV filter you might have attached to the front of the lens.

Unfortunately, killing the fungus is not removing it. If it does not affect image quality, then that might not be a short-term concern. But you'll likely need to continue to expose the lens to UV light regularly. Since fungus is a spore, it can lie dormant for very long periods of time, then flourish again when the conditions are favorable. Those conditions are heat, moisture, and protection from UV light. Just another reason to remove any UV filter you might have on the lens for no real logical reason.

Beyond that, some lenses make it fairly easy to remove the front element from the front. Depending on how your lens is constructed, you might be able to do that, clean the back of the front element, along with the front of the next element, and put it all back together relatively painlessly. I've done it with a Tamron SP 17-50mm f/2.8 Di II. Just be sure to mark the position of each piece relative to the piece it is attached to before you loosen anything, particularly anything that looks like an optical adjustment.

For some hints on how lenses are put together and how to do simple cleaning of the easy-to-get-to internal parts of some of them, take a look at this blog entry from Roger Cicala, the founder and chief lens guru at lensrentals.com.