CS (for now) is still accessible via the L&S pathway. Go through three extremely hard classes CS61a, CS61b, CS70 and a couple other requirements as well. Get above a 3.3 GPA (though this might change to 3.5 or 3.8 from what I've heard).

All of this assumes that the pathway will still be open next year or while you're trying to declare. There is a lot of change going on in the major right now especially around reducing enrollment in an extremely crowded major. I will caution you on 2 things:
1. CS is extremely hard to do and it's only getting harder. To maintain a 3.3 GPA on the curve you will need to be in the top 30-40% of your class. The courses are designed to make sure the majority of people cannot declare in order to save space in the major and the people who make it are generally those who already have an extremely thorough math or CS background.

1. Things are changing and the future of the L&S pathway is uncertain. There is a serious risk of while you're attempting to do CS the pathway might just close off. That's why I said the (for now). The department is reconsidering how it admits people because the L&S pathway has started getting extremely crowded and you can only so fail many students in the weeder declaration classes before things start breaking.
   As someone who's almost done with the CS degree I really advise you to be careful about doing CS.