You can view the Change Log information for a given app in the App Gallery, when the app in question is already installed and an updated version is available to upgrade to - you would also be able to see the 'Update' button. If you have already upgraded the app in question, then you can query the APEX_PKG_APP_AVAILABLE view to view the associated Change Log information. For example, for P-Track (app ID 7220):
select change_log from APEX_PKG_APP_AVAILABLE where app_id = 7220
I hope this helps.