You can generate a new Extension Key by doing the following:
1. Sign in to your customer account on Client's Portal.
2. Under Extension Keys tab, click Add New Extension Key.
To associate the new Key with your current IP/Domain, please complete the steps:
1. Copy the newly generated Extension Key.
2. Paste the Key in the Admin Panel of your Magento: System > Configuration > M2E Pro > Billing Info (for Magento v1.x) or eBay/Amazon/Walmart Integration > Configuration > Settings > License (for Magento v2.x).
3. Confirm the changes.
4. Click Save Config at the top of the page.
5. Clear Magento cache: System > Cache Management > Flush Magento cache and Cache Storage (for Magento v1.x) or System > Tools > Cache Management > Flush Magento cache and Cache Storage (for Magento v2.x).
Read why a new Extension Key must be obtained after Magento is moved to a new server.