Go to Applications/Utilities/Keychain Access. Select the appropriate keychain (possibly in Login Items?) (You can also use KA's search field for Digital EU certificate if you can't find that keychain in Login Items), find the Digital EU Certificate and double-click it. You should get a window that has the application name and check "Show password". Enter your administrator password in the window that appears so that your password will show. Then, you can change it. Alternatively, in Keychain Access, try the Help menu.
I hope that this is useful.