To renew your eBay token you will need to navigate to your eBay channel settings. Go to Settings > Sales Channel Integrations > eBay.

At the top of the page you will see a button that reads "Renew token". Click this.

 
An eBay window will pop up after you click the "Renew token" button, prompting you to sign into your eBay account. Provide your email/username and password to login. This will automatically renew your eBay token.