How to prepare for undergraduate research of which I have no background in?
My advice is to discuss this matter directly with whoever your supervisor will be -- tell them about your past experience, your concerns, and ask for their advice. They may be able to have you shifted -- or they may tell you that they were explicitly looking for your biology background. 

As to why they would be explicitly looking for your biology backgrounds -- computer scientists frequently collaborate with other scientists when working on large-scale simulations. The reason for this is that there's two sides to the simulation -- the actual code required to run it, and the domain knowledge required to design the code in the first place. As @Sumyrda pointed out, this may be one of the goals of the internship (to give you experience in working with/as the domain expert).