Many service shops, though including some at dealerships that sell cars with “lifetime” coolant say you should do a coolant change more often than the maintenance schedule recommends, such as every 30,000 or 50,000 miles.
A typical mechanic will recommend changing coolant every 30,000 miles. But many will tell you, changing the coolant is not even on their radar. An owner's manual might recommend changing the coolant/antifreeze after the first 60,000 miles, then every 30,000 miles.
Coolant flushes remove rust and scale deposits, which build up over time. It's important to get rid of these deposits because they can cause overheating and damage your entire cooling system. Getting a coolant flush will prevent the old anti-freeze from becoming acidic.
You are fine just topping off your coolant. Check it every once and awhile because you really shouldn't lose any coolant unless its leaking or being burned up in the cylinders (highly doubt either of these issues though). That's fine to add coolant to the expansion tank.
Open the hood and locate the engine coolant reservoir. If the coolant level is low, add the correct coolant to the reservoir (not the radiator itself). You can use diluted coolant by itself, or a 50/50 mixture of concentrated coolant and distilled water.