Wherever you live, whether an apartment or house, the first and simplest thing to do is to seal the air leaks. "If you have an older house that's never been air-sealed, this may be a big part of your heat gain problem, especially if you have leaks from the attic. If you haven't had a blower door test, get one."
I've seen dressers, piles of clothes, and dog beds completely or partially covering supply or return vents in homes. Choking off air flow at the vents increases the pressure in the duct system and reduces air flow.
Getting a smart thermostat will increase efficiency and keeep your bills lower.