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.