I'd keep an eye on it and the fluid level, if it's only leaking a little bit I probably wouldn't worry about it till next time I was doing a service and wanted to change the coolant
That said if it's leaking at any sort of rate you'll want to be in there, it should be a case of draining the coolant and unbolting the water pump, from there you could just put a new seal in after checking all was well I'd have imagined. Not done it myself though.