You absolutely do not want to steam leather. You will shrink it and when it dries, it will be stiff as a board. The only thing that removes mildew is bleach. You can use Tilex, which has about a 10 to 1 bleach solution in it, followed by a clean soft towel. You need to let the Tilex sit on the surface for a while to let the bleach kill the mildew. You may have to repeat the treatment a couple times before the mildew is all gone. Then follow that up with Lexol leather cleaner followed by Lexol conditioner. These come in spray bottles also and are available on line separately and in kits.
No one lives forever, the trick is creating something that will.