Call the restaurant, ask for the owner or manager and tell them. The majority of restaurateurs want to give quality and will try to make it right. It's fair to at least give them a chance to do that. No restaurant is perfect.
If they are rude to you, and you ACTUALLY have a real, valid issue you can describe in detail, then your only option is to go public on TripAdvisor or whatever.
PLEASE NOTE that this is all predicated on your having an ACTUAL issue with the restaurant, not something like food being cold or not the right consistency – it IS takeout after all, and many restaurants hate doing takeout for that reason. So be sure that your complaint is truly valid before you vent online.
