CangesCagnes-sur-Mer has a railway station, with frequent trains from Nice (generally every half hour). Trains take about 15 minutes, and you just buy the ticket at the station on departure. Depending on where you are in Nice a busbus 94 might be useful too. The local bus company has its website here: http://www.lignesdazur.com/ (Press the British flag for English).
Generally speaking:
Getting there by public transport is easy and cheap.
Doing it as a day trip perfectly feasible.
Haut de Cagnes is about 1km from the train station. If you really don't want to climb, you can walk to the town center (square Bourdet, also a stop on bus 94) and take bus 44 uphill.