How do I change radiator fluid

Depending on where you live, it might be better to take it into a shop that has a coolant recycling machine. They can change the fluid, recycle it and backflush the cooling system in one operation. If you want to go it alone, you'll need to get a cooling system flush kit which will give you a 'tee' to put in the heater core return hose, 2 hose clamps, a coupler to connect it to your garden hose, a cap to put on the hose thread port after you're done, and a can of somewhat caustic cleaner. Also be sure to have a large drain pan (at least 5 gallons), and containers to put the old coolant and the first bit of flush water in.

Follow the instructions on the flush kit for adding the cleaner and warming up the engine, and such.

Drain radiator as completely as possible into drain pan, empty drain pan into a container (lidded 5 gallon buckets are good for this). Put drain pan back under, and leave radiator cap off.

If you haven't already installed the tee in your heater hose, do so now.

Attach garden hose to tee, using the coupler that was provided in the kit, start hose at a fairly low rate of flow, start engine.

When the fluid coming from radiator is fairly clear (no more antifreeze color), you can remove the drain pan and increase the hose flow by a lot. Let water flow through for about 5 more minutes, then stop engine and stop hose.

Remove hose, put cap on tee. Allow water to drain out of radiator as much as possible. Close radiator petcock. Refill coolant, you can mostly use pre-mixed, but there will usually be a little water stuck in the engine that dilutes it, so you might want to start with a liter or two of pure antifreeze, then fill to capacity with pre-mixed. In most climates a final ratio of water to antifreeze can be anywhere from about 30/70 to about 70/30, in really cold climates you'll want to use the ratio that is indicated for your lowest winter temperatures.

Dispose of the old coolant and the first bit of flush water in accordance with whatever regulations may exist in your area, the stuff is poisonous to animals and to septic and sewage treatment bacteria, so not down the drain. Where I live it has to go to a household hazardous waste disposal site.