Question

how does enzymes speed up chemical reaction?

Answer 1

Enzymes decrease (or increase) the energy needed for the chemical reaction. Therefore, the reactions are sped up drastically.

Answer 2

Enzymes decrease the activation energy for a reaction which is why the reaction speeds up.
If they increased the activation energy then the reaction would slow down.

Answer 3

Enzymes are a form of protein that do two major roles in living things. Proteins build structures like muscles and bones, while enzymes control chemical reactions at a cell level. The reason they can do this is what @surry99 said (Just giving additional information so you know)