How would you change the default avatar in game to something else?

This is just an example of resizing a players head when they join the game. I'm not going to give you the exact code, but you should be able to alter this to your liking. 1 up this answer if it helped!

while wait() do
players  = game.Players:GetPlayers()
for i=1,#players do
char = players[i].Character
if true == true then
char.Head.Size = Vector3.new(10,10,10)
end
end
end

Hey, Idk if this is what you would want but I just made this script for the first time. I am not so sure on it it works on how you would want it to work but it does work..

- Make a Part with a size of (6,6,6) and put it in ReplicatedStorage.
- Name that part 'Cop'

while wait() do
    local Part = game.ReplicatedStorage:WaitForChild('Cop') --Name of part ;)
    local plrs = game.Players:GetPlayers()
    for i = 1, #plrs do
        local Char = plrs[i].Character
        local Torso = Char.Torso

        local clonedPart = Part:Clone()
        clonedPart.Parent = Torso

        local Weld = Instance.new('Weld', Torso)
        Weld.Part0 = Torso
        Weld.Part1 = clonedPart
        Weld.CFrame = CFrame.new(0,0,0)
    end
end

If you want to use a custom model and a cube is just an example, you need to do something hard, to be honest. An easy way is morph system, which copies a model to your character, welds them and makes your character's parts invisible. If you are looking for a more complex way, you can build a default character, make them controllable with keyboard input, manipulate camera and change it's type to scriptable and change position and rotation of the camera.

My suggestion is, if you are going to use second way, you can make your character move with animations. To do that, put a HumanoidRootPart in DefaultCharacter and connect it to other parts with Motor6D objects, so your character will move with animation even if it is a spider or a horse :)