The postal address that Google shows for a business in the search results comes from the listing for that business that Google shows on its maps.
To correct that address, claim your business listing in Google My Business. Here are Google's instructions for claiming and verifying your business. Their help document for editing your listing has instructions for changing the address:
Enter the complete and exact address for your business location. Read our address entry guidelines for more recommendations.
Note that changing your address in between the postcard request and verification code entry stages of verification will cause the verification process to reset.
You can also report a problem to Google through maps and hope they check it out and fix it soon. I don't believe that reporting problems is usually very effective compared to claiming the business listing though.