Go to the Digital Signatures tab and click on an entry in the signature list to highlight that entry. The ‘Details’ button will become enabled, and when you press it and wait for several seconds you will get a message telling you whether or not the digital signature is OK. If it’s not OK, you need to download a fresh copy of the installer file (and if it is OK, the problem lies elsewhere and there’s no point in downloading the installer file all over again).