2KClO3(s) => 2KCl(s) + 3O2(g)
is there an increase or decrease of entropy?

Entropy increases

Explanation:

Entropy (S) is a measure of the degree of disorder.  For a given substance - say water - across phases the following is true ...

S(ice) < S(water) << S(steam)

For a chemical process, entropy changes can be related to increasing or decreasing molar volumes of gas from reactant side of equation to product side of equation. That is ...

if molar volumes of gas increase, then entropy increases, and

if molar volumes of gas decrease, then entropy decreases.

For the reaction                  2KClO₃(s) => 2KCl(s) + 3O₂(g)

molar volumes of gas =>       0Vm*            0Vm      3Vm

*molar volumes (Vm) apply only to gas phase substances. Solids and liquids do not have molar volume.

Since the reaction produces 3 molar volumes of O₂(g) product vs 0 molar volumes of reactant, then the reaction is showing an increase in molar volumes of gas phase substances and its entropy is therefore increasing.