UT Austin - Undeclared CNS

Hi, Need some help. My Daughter an international applicant got accepted to UT Austin - Undeclared CNS. She is strong across STEM and was not sure if she wants to take Chem / Bio or Biochem and hence applied undeclared.

How does the system work for undeclared, by when she needs to declare the major.

Assuming she wants to move to engineering, how difficult is it? She is academically sound and strong in Math and science.

Thanks in advance