Changing your lower units lube seasonally is a must to extend your outboards life, and its real easy to do.
First drain the old lube.
On your motor you will find 2 slotted fittings that are the top and bottom lube plugs. You can see them here.
Place a pan under the motor and remove the bottom plug. You will see the old lube start to dribble out as is seen here.
Next, remove the top plug to break the vacume holding the lube in the motor
The oil will now flow freely. Allow time to totally drain.
Next refill the lower end lube. This can be done by using a squeeze bottle as is shown here or using a pump that is threaded on the lube container. Fill unit with lube until it comes out the top hole.
While holding the bottle or pump fitting in place, replace the top port plug and tighten. This will once again create a vacume allowing you to remove the pump fitting or squeeze bottle without loosing fluid.
Finally, replace the bottom plug to complete the job.
Thats all there is to changing your lower unit lube.