Step 1: Click on your Profile Picture and then my blog option.
Step 2: Click on the Blog Dashboard from the dropdown.
Step 3: The Blog Dashboard will appear like this.
Step 4: Scroll down on the Dashboard, and click on the Advanced Tab option.
Step 5: In the Advanced Tab, when you scroll down to the bottom you will see the "Convert this blog to a team blog" section. Click the "Convert to a team blog" button to convert your personal blog to a team blog.
Please Note: This step is irreversible.