To update your MWS credentials, log into Seller Central and navigate to Apps and Services > Manage Your Apps:
You should then see a list of Applications, one of which will be "ZentailCommerce", which will likely show Expired:
To the right of the App expiration date, you should see a button labeled "Renew" and you should see a new value appear in the "MWS Authorization Token" field. This is the new value you'll place in the Seller-Developer Auth Token field on your Amazon Integrations page.
While you're doing this, the Repricer token should usually be updated at the same time. The repricer should be in the App list as well, named "ZentailRepricer" and you can do the same with it as you did with the regular MWS token, adding the new value to your Amazon integration.
Be sure to click the blue "Save Settings" button in the upper right corner on your Amazon Integrations page and your Amazon MWS link should be reconnected!
If you run into any problems or have any additional question, feel free to chat in or email [email protected] for assistance at any time.