How to make a car despawn after you spawn in another one? (Like in vehicle Simulator)

You could try using a variable that references the old car, and each time the car spawns the old one is deleted.

It would look something like this:

local currentCar --creates new variable that is "nil" (doesn't have a value)
script.Parent.MouseButton1Down:Connect(function()
    if currentCar then -- if there already is a car
        currentCar:Destroy() -- then destroy it
    end
    local clone = car:Clone()
    currentCar = clone
end)

This is just an example so you would need to create the rest of the script by yourself.

this will only do things on the client side, lets move it to the server

local script

script.Parent.MouseButton1Down:Connect(function()
game.ReplicatedStorage.spawncar:FireServer()
end)

server script

game.ReplicatedStorage.spawncar.OnServerEvent:Connect(function()
if game.Workspace:FindFirstChild("car name") then
game.Workspace.car name:Destroy()
else
game.ServerStorage.car name:Clone().Parent = game.Workspace
end
end)