Water drains when the motor is down. For extra peace of mind, turn the flywheel by hand or do a short crank to ensure water caught in the water pump is pushed out.
Remove the fill and vent screws and let the oil drain into a catch container, then look for water in the bottom of the container. If the oil looks good, fill the lower unit from the bottom.
On a scale from 1-10, changing outboard lower unit oil is a 0. One of the easiest jobs on a boat.