If you have floating licenses, you have to connect or go to the server where there is the license server.
In the program, there must be a button to update it. Or you have to deactivate the license, then reinstall the 2015 version software (available on the DVD or the website) and reactivate the licenses via the internet.
Indeed, when using a license server, you must not only update solidworks but also the license server.
For this you must have the rights to this server. Then you go to the solidworks installation manager and you select "Server Product". The following windows will allow you to update the server.
From memory, you have to restart everything once updated for this to be taken into account.
Thank you for the answers; It was necessary to update the license manager and above all not to forget the re-activation of the license on the "Server administration" tab.