Developer licenses can't be renewed, but you can apply for a new license every 6 months starting one week before the expiration of your current license. If you try to apply for a new license earlier than this, you'll get re-sent your existing valid license instead.
To renew a 10GB Developer license, use the same form as your initial application at: https://dev.splunk.com/enterprise/dev_license - please keep in mind that this type of license request is approved manually, so it may take 2-3 business days before you get a response. If it's been longer than that, please reach out to firstname.lastname@example.org
To renew a 50GB Dev/Test license, use the same form as your initial application at: https://splunk.com/dev-test - since this type of license is only available to paying Splunk customers, if you have any issues with this type of license request, please reach out to your Splunk sales representative.