You can't adjust the Focal Length of you telescope. A barlow lense will double the focal length and a focal reducer will about cut it in half, but that's not the issue. As the other post said, with higher magnification you loose sharpness. There's nothing you can do short of getting a bigger scope. Also, Jupiter and Saturn are getting lower in the sky now, that makes it harder to focus to. My advise is to enjoy some other items that are higher in the sky and not to worry about real high magnification. Remember, it's not about magnification, it's about clarity and sharpness. That will come, with most items, with low magnification.
That's it;
BOB SAL