How can the order of precedence in this formula C12 C13F4 be changed so that cells C12 and C13 are added together as the first operation that occurs?

You put parentheses around the calculation that you want to do first. The operators disappeared from the question, but here is an example. In the expression:

1 + 2 * 3

the multiplication is usually carried out first. Therefore, the result is 7. If you want to add first, you write it as:

(1 + 2) * 3

In this case, the addition is done first, and the result is 9.