I ordered the N9051A, but it is no longer installed on the signal analyzer. The N9051B is installed, but my N9051A licenses don’t work. What shou

The N9051B is the recommended replacement. The N9051A license can be upgraded by adding a low-cost N9051B-A2B license.   More information can be found in this FAQ:  If I have N9051A Pulse Measurement Software, how do I upgrade to N9051B?

If you need the N9051A version, you need to install the N9051A on the instrument.  The N9051A software file can be found at this link:  N9051A Pulse Measurement Software Downloads   Download the software, run the installer.  It is recommended  you remove or uninstall the N9051B software first.