Authentication Manager licenses are stackable, as long as the license numbers are the same between the old license number and new license number.

Before applying the new license

1. Login to the Authentication Manager primary server's Security Console as a super admin user.
2. Navigate to Setup > License > Status and click View Installed Licenses. 
3. Click on the context arrow next to the License ID number and choose View. 
4. Compare the license serial number visible in the Security Console to the license number in the license .xml file you received, specifically the license:customerAccountId value.
5. If the license numbers are the same, the new license can be installed immediately.  Licenses for Authentication Manager are stackable and the new license will be added on top of the existing license.

If the license numbers are the same

If the license numbers are the same, the new license can be installed immediately.  

1. Within the Security Console select Setup > License > Add New.  
2. Browse to the directory containing the new license.xml file and select it.
3. Click Next then verify that the information regarding your license is correct.
4. Click the Install button.

If the license numbers are different

Review article 000046582 - https://community.rsa.com/s/article/The-License-serial-number-being-installed-doesn-t-match-the-license-serial-number-stored-on-the-server-when-installing-an-RSA-Authentication-Manager-license before continuing.