To drain the coolant, you pull the plug from the bottom of the radiator,
which is embedded into the bottom of the radiator, to get it out, you'll
need a #2 Phillips screwdriver. Once you've screwed that plug out, pull
the radiator cap off and let it drain.

There is a trick to filling the coolant, there is a bleed screw somewhere
on the motor. This will let the air bubble that occurs in the head (and
possibly the throttle body). This screw is an 8MM little screw under the
distributor mounting on the Altima. This is at the back of the head/block.
Once the bleed screw is pulled out (and the plug installed into the
radiator), fill the radiator until the fluid flows from the bleed whole.
Then replace the bleed screw, tighten it. Not too much, it's not the
strongest screw. Then fill the radiator to it's top. Then fill the
reservior to the midpoint of the MIN/MAX marks. Then run the vehicle to
operating temperature, with the heat on and the fan on a medium setting.