To solve the equation
\[ \frac{h}{2} + \frac{h}{3} = 1, \]
we first need to find a common denominator for the fractions. The denominators are 2 and 3, and their least common multiple is 6.
Next, we'll rewrite the fractions with a common denominator of 6:
\[ \frac{h}{2} = \frac{3h}{6} \quad \text{and} \quad \frac{h}{3} = \frac{2h}{6}. \]
Substituting these back into the equation gives:
\[ \frac{3h}{6} + \frac{2h}{6} = 1. \]
Now we can combine the fractions on the left side:
\[ \frac{3h + 2h}{6} = 1 \quad \Rightarrow \quad \frac{5h}{6} = 1. \]
To eliminate the fraction, multiply both sides by 6:
\[ 5h = 6. \]
Now, divide both sides by 5 to solve for \( h \):
\[ h = \frac{6}{5}. \]
This can also be expressed as a mixed number:
\[ h = 1 \frac{1}{5}. \]
Thus, the time it will take Kelly and Nina to decorate the cookies is
\[ \boxed{\frac{6}{5}} \text{ hours} \quad \text{or} \quad \boxed{1 \frac{1}{5}} \text{ hours}. \]