For Italian clik here

This procedure is only for themes and of course for people who have not original symbian certificates for their phones or did'nt hacked the phone ....

First install on your phone SignSis

With a file manager create on your memory card a new folder (for example Themecert)

In this folder insert key and cer (clik here to download) and theme that you want to install with certificate expired

Now open SignSis:

select the theme you want to certificate and select key and cer from your new folder (you don't need to put password)

Now clik on Options and select "sign" 

In your folder now there is a sisx file with a new certificate and you can install it with no problems