If your previous Informant was a subscription based Informant then the Informant 5 update is available to you for free. Install Informant 5 and in the settings tap restore purchases.
If you were subscribed to Informant Sync you can install Informant 5 and use it for the duration of your Informant Sync subscription for free. Just set up your Informant Sync account in the settings and tap begin sync.
If your Informant is a standalone purchase ( not a subscription and with no Informant Sync account) then Informant 5 will be a paid upgrade.