Best Buy Co. said Chief Executive Officer Brian Dunn resigned and board member G. Mike Mikan is taking the position on an interim basis as the electronics retailer focuses on smaller stores and Internet sales. The change was a “mutual agreement’’ that new leadership was needed, the Richfield, Minnesota-based company said today in a statement. A committee of directors has been created to search for the next CEO, the company said. Best Buy, the world’s largest electronics retailer, has been cutting jobs and shifting to smaller store formats amid competition from Internet retailers such as Amazon.com