Navigate to Marketplaces > Marketplace Subscriptions.

You'll then want to click the 'x', removing your current token.

You should see a notification that your token has been cleared.

After refreshing the page, your 'Authorized?' Check box will be red. Click the Authenticate button to establish a new token.

You'll be taken to an eBay login screen. Use your eBay login credentials here.

If your login was successful, you should get a notification saying 'Authentication complete' and a green checkbox under 'Authorized?'