If you've received a message that your eBay token is going to expired or has already expired, the steps to take to renew it is to go to Setting -> Channels and under your eBay section for that instance, you will see the Token field: 

When you click "Revoke Token" you will be asked if you are sure about your decision. After you go through that and refresh the page, you will see a button to "Authorize eBay."  Click that button and then you will be sent to eBay where you just follow the prompts.