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.
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​Please Note: This step is irreversible.