If you have bought a different license key from the one that you have installed on the PBX then you need to take a backup without the License key information uninstall the PBX and re-install with the correct key.
Note that before you re-install you will need to login in to the ERP https://erp.3cx.com with the credential you have in the email you received at the initial install and release the previous key from the FQDN.
If you upgraded the free key then all you have to do is click on the license Tab in the information section of the management console then click edit and ok without changing anything and your upgrade will be applied.