Take the car apart and remove all the parts until all you have left is the metal shell.
Put all of the trim, interior etc in a sealed sandwich bag for safekeeping during this operation.
You could just spray over the existing paint but you’ll have much better results if you strip the existing paint. This is because factory paint is thick and uneven. Spraying over may result in loss of details or uneven gaps or thicknesses. It’s easier than it sounds. Put some brake fluid in a bucket and then put the car body in it. It make take a day or two but the brake fluid will remove the paint. (You may need to hit some areas with an old toothbrush or other brush just don’t use anything that will scratch the metal.)
Once all of the paint is removed, wash and dry the body to get the brake fluid off of it. (You may want to use the brush again for the out of the way places.)
Try not to handle the body before painting and between coats of paint because the oils on your skin can affect the paint. (Use a coat hanger bent to hold the body so you don’t have to touch it.)
You can use any kind of spray paint to repaint the car but you should prime it with a primer. Most are a light gray. Just about any kind of spray paint will work, but make sure that your primer and final color are compatible! (For example you want to use enamels for both the primer and final coat etc.)
When you paint keep the can about 12 inches or 30 cm away.
Start by spraying ahead of the body. Then move in a gentle sweeping motion sideways. Note: Move in 1 sideway direction.
Don’t concentrate the paint as this will lead to runs, and will result in too thick paint and spoil the details on the body. That’s why you hold the spray paint 30 cm away.
Pointing a can at the body and spraying will lead to splattered paint and will not make for a good paint job. That’s why you should start ahead of the body and move past it in a gentle sweeping motion sideways).
You will not cover the entire body in the first try.
Wait for a least 12 hours between coats. (Warning: Try not to handle the body between coats of paint because the oils on your skin can affect the paint). Use a coat hanger bent to hold the body so you don’t have to touch it.). However, if you really mess something up you can always get more brake fluid and start over.