To shift the graph of the function \( f(x) = -x^2 \) up by 5 units, you need to add 5 to the function. Thus, the equation of the new function \( g(x) \) would be:
\[ g(x) = -x^2 + 5 \]
This function represents the graph of \( f(x) \) lifted upward by 5 units.