Go to: Cisco Software Central → Smart Software Licensing (for new) or Classic License Registration (for ASA 9.15 and older).
For Classic licensing:
When purchasing a license for a Cisco ASA, you typically receive a Product Authorization Key (PAK). This is an alphanumeric string that must be registered on the Cisco Licensing portal (cisco.com/go/license) to generate the actual license key file. cisco+asa+license+key+generator+install
license install <license_filename.lic>