You can generate the license by accessing https://my.atlassian.com using your Atlassian account account. To generate a new license perform the following:

1. Navigate to https://my.atlassian.com and login using your Atlassian account
2. Click on New Evaluation License to generate the license
3. Choose the product from the dropdown menu and provide your existing server ID
4. Agree to the terms and conditions and generate the license

Once the license is generated, update it in your Bitbucket Server instance by performing the following:

1. Navigate to Bitbucket Server Administration >> Licensing
2. Click Edit License
3. Copy the new license and click Save to store the changes