Options include intramuscular injection, slow-release implants, and skin creams. I prefer skin creams, because you get a ~normal diurnal cycle. And old-school cream, not Androgel or Testim. Because it's much less expensive, and leaves ~no residue.

If you actually have low testosterone, many doctors will prescribe supplements. It may take some work to find one who doesn't consider age-related decline to be "normal", and so not warranting treatment. And if you can't, then you'll need to shop online.