1. Draining the fluid. There are 3 drains for the common transmission/hydraulic/differential fluid sump (under the rear axle, under the hydraulic pump and under the transmission). There are 3 drain plugs, but only one reservoir.
2. After you've drained out about as much as will come out, put the drain plugs back in and pour in about 2-3 gallons of diesel fuel. Open the inspection covers on both sides of the tractor and reach in... use your fingers and/or a parts cleaning brush to loosen all of the accumulated sludge in the bottom of things... Stir it all up real well.
3. Open the drain plugs (in the order listed above) and drain out the 'dirty' diesel.
4. Let it sit for 15-20 minutes, or so, then pour the diesel BACK into the reservoir (after replacing the plugs, again) stopping before the settled sludge pours out, and repeat steps 2 and 3.
5. Do this 4, 5, even 6 times (until you've got most of it out) then put the plugs in for good, put your gaskets on your inspection covers, and pour in your 5 gallons (really about 4½ gallons) through the one filler plug on top of the transmission shifter. You can determine the 'proper' fill level by leaving the bottom bolt out of one of your side inspection covers... and fill until the oil starts to run out of that hole.
You should use GL-1 specification oil (90wt Mineral Oil) in the Ford N tractors. Personally... I get mine at Tractor Supply... this is what their 2 gallon bottle looks like.