How Can I Replace Water On A 65horsepower Mercury Outboard Motor?

There should be no water in an outboard motor other than a small residual amount left in the cooling system which is normal and doesn't need draining or replacing unless your motor will be stored in freezing temperatures.

Hopefully the motor has no water IN it.

It is OK for the motor to be ON a boat IN the water.
It is OK for the motor to be IN a boat ON the water.
It is OK for the motor to be ON a boat ON the water.
                              And finally
It is OK for the motor to be IN a boat IN the water.
The point is:
                     ---No Boat NO FLOAT --

It is also OK for the motor to have water ON it; however, this can cause wetness and the water must be changed every 30 hours or so of run-time. This may be accomplished with a common garden hose attached to a water source. Or. If at all possible,  by submersing entire assembly [boat + motor] in the nearest body of water.........