I want to change my major to art history, the program requires that I take 2-4 studio art classes. My question is how do they grade in these classes? I’ve never been an artist, but I do like learning about art.
My experience has been that classes are graded on ability just like other subjects. I’ve heard a few comments over the years where people think they were unfairly graded in art classes because they claim, “Well, I tried. I worked really hard.” Well, so does the kid who sucks at math because their brain doesn’t work that way. 
In other words, don’t expect and easy A if you don’t have talent for art.
Hmmm okay I’ve always been really bad at anything involving a pencil lol should I consider a different major?
I’d look into the specifics of what 2-4 courses are required. Does photography or printmaking cover the requirement? A creative mind and eye can still be required but they are more mechanical in nature in the construction. If it concerns you, you can talk to the department head or representative to express you interest and concerns and get their input. I’d also look around at the requirements of different departments at different colleges. I just looked at the major requirements at two Art History departments that I know to be very well regarded. One of the colleges requires 1 fine arts introductory course where no previous experience is required. The other does not require fine arts coursework for their art history majors. Lastly, wouldn’t worry about a few Bs or such in college if you are taking some courses that will further you along the path you desire.