By bus
City buses arrive at the square from the main central streets; you pay with the same Metromoney card as on the metro.
A bus is useful if you want to see part of the city en route, but it depends on traffic.
By taxi
Taxi apps show the price up front, which makes them more predictable than hailing a car in the street.
For a short distance a taxi is convenient, although at peak hours it can take longer.
Which to choose
If time matters, take the metro. If you have luggage or are travelling at night, take a taxi.
Either way, reaching the square is easy because it sits in the middle of the centre.