If you are assigned as a reviewer on a proof, then you have the ability to make a decision on the proof. 

To make your decision, open the proof, navigate to the Review tab and select either the green check mark (Approve) or the red x (Reject). 

If you need to change your decision, click the arrow to the right of your previous decision.

Note: Only one reviewer assigned to each review stage is required to make a decision. If you need each reviewer to make a decision, then you will need to create a review stage for each reviewer.