Newb in Need

Hello, I want to start with a warning that I am completely new to the world of atv and no absolutely nothing about them so at the risk of sounding special here it goes…….
So I just bought a used 2000 Sportsman 400 from a private sale, When he delivered it to my house he stated that if it didn’t go into gear to turn the idle down as he turned it up because it is winter and cold. When I bought it he drove it right into my garage, So yesterday I started it (5 days after I bought it) and it started right up, I put it into gear and pulled it out of my garage, I then parked it for a minute then put it in reverse to put it back into my garage, it seemed like it didn’t want to go into reverse because it seemed to be rev’d high. once in reverse it kind of drove itself backwards without any accelerator very very slowly. Now it is stuck in reverse and it will not go into neutral or drive. I am unable to find the idle screw to adjust it as I don’t know what I am looking at, I did many searches online but have had no luck. Does this sound like a problem that would occur from a high idle or would it be stuck in reverse for another reason? and does anyone have any links with a video or a pic showing what the idle screw looks like and is located? Thanks everyone