Step 1) Login to your Google My Business dashboard.
Google Business Click Here
Step 2) In Google My Business, click on the 'Manage Location' link towards the bottom of the page over your business page preview.
Step 3) Click on the 'Reviews' tab at the top of the page to show a list of reviews.
Step 4) Find the review you wish to reply to and click on the 'View and reply' link.
You can now directly respond to the review.