You cannot change the venue and meals in the selected package; you can change them by choosing a different package. On the bottom panel, you see icons representing services: photographer, party host, decor, musicians, etc. Click on the plus sign at the corner of an icon to add options to your package. If you want to remove an option from the package, press the cross.

PS: In some packages, it may be impossible to add some options like decoration or musician because the venue does not allow it.