How do I make a model rotate like a fan?

You do not have a simple rotate command, also rotating a model is not the best way to do what you want, you should union the parts you want to rotate first.

To rotate them change the CFrame, not the Vector3 orientation as it might teleport ontop.

CFrame.new(Vector3 pos, Vector3 lookAt)

Or use another way, just use cframe xD

To keep rotating it use a loop

local part = script.Parent
n = 0

repeat
    wait(0.2)
    if n <= 360 then
        n = n + 1
    end
    local rot = Vector3.new(math.rad(part.Orientation.X),math.rad(part.Orientation.Y),math.rad(part.Orientation.Z))
    part.CFrame = CFrame.Angles(math.rad(part.Orientation.X),math.rad(n),math.rad(part.Orientation.Z))
until false

I used CFrame.Angles in the example that looks like to work better for me, you can see how it works for you though, you can also use tweens.