You can change which credential appears on your answer in two ways.
Before your answer is published, you can click the "Edit Credential" link under your name in the answer editor:
After your answer is published, you can click the "Edit Credential" link in the "..." menu:
Tip: If you want to update the default Credential used for all answers, go to your Profile page and click on the "Edit" link next to the Credentials & Highlights section. Then, click on the "Edit" link to the right of the Credential you want to use and select "Set as Default".