To upgrade the current license of your cisco asa firewall, you need to order a new license key from Cisco at www.cisco.com/go/license. You will receive a new license key in your email after a couple of hours. This license key is a five element hexadecimal string in the form 0xffd8624e (as an example).
To apply this new license key in your security appliance, configure the following:
ASA5500(config)# activation-key 0xffd8624e
ASA5500#copy running startup