You can obtain a new Extension Key on the Clients Portal by following these steps:
- sign into your customer account on Client's Portal;
- under the Extension Keys tab, click Add New Extension Key;
- copy the newly created Extension Key;
- insert it in the Admin Panel of your Magento: in System > Configuration > M2E Pro > License (for Magento v1.x.) or eBay/Amazon Integration > Configuration > Settings > License (for Magento v2.x);
- Confirm the changes;
- click Save Config at the top of the page;
- clear Magento cache in System > Cache Management > Flush Magento cache and Cache Storage (for Magento v1.x.) or in System > Tools > Cache Management > Flush Magento cache and Cache Storage(for Magento v2.x.).