Step 1:  Login to your Blogger Account and open your Blogger Dashboard.

Step 2: Click on settings.

Step 3: Select Basic as shown below.

Step 4: Under Publishing, go to the Blog address and click on Edit as shown above

Step 5: Go to "Advanced settings", You will find an option saying “Redirect yourblogname.com to www.yourblogname.com” .

Step 6:  Select that option and click the "Save" Button

** If you need help kindly submit a Support ticket here.