Click on Integration > Your Amazon Marketplace > API Settings.
You'll find the option to Update token there. Scroll down to learn how you can Renew your MWS Authorization on Amazon.
Copy and paste the Amazon MWS token exactly as it appears on the page. Make sure there are no leading or trailing spaces, as any extra spacing will give you errors.
If your token has changed, then you'll need to update the token on SellerZen. Otherwise, click on the Resume button:
Renew your MWS Token on Amazon
Go to Settings > User Permissions. Click on Manage your Apps.
Find SellerZen and click on Renew. Copy and paste the new MWS Token in the SellerZen field above if necessary.
If the token has already expired, then you may have to click on "Enable."
Follow through the prompts on Amazon to renew the token.
You may need to go back to SellerZen and Resume integration on the marketplace.