Converting to a 3-way switch with a 2 wire cable
Best way would be to use a 'smart' switch of some sort. There are many on the market. Some are wireless (radio frequency), some use signals on the powerline. These switches require hot and neutral.

The downstairs one would be wired as a normal switch and it would directly control the light. The upstairs one would just be configured to control the other switch, and the two conductors would carry hot and neutral.