How to change velocity in bodyvelocity?

The X, Y, and Z properties of Vector3 are read-only, meaning you can't edit their values. But the velocity property itself isn't read-only, so we can edit that. It is, however, a Vector3 value, so we have to use Vector3.new.

script.Parent.Parent.M1.BodyVelocity.velocity = Vector3.new(0, 0, 2)
wait(3)
script.Parent.Parent.M1.BodyVelocity.velocity = Vector3.new(0, 0, -2)

To keep this repeating, we must put it in a loop. The while loop will suffice. This loop takes the form of,

while (condition) do

end

with the loop only running if condition equals true. Since we want it to repeat forever, we can just use the word true, since true will never equal false.

while true do
    script.Parent.Parent.M1.BodyVelocity.velocity = Vector3.new(0, 0, 2)
    wait(3)
    script.Parent.Parent.M1.BodyVelocity.velocity = Vector3.new(0, 0, -2)
    wait(3)
end

You can't set individual properties of a Vector3 by themselves, you have to set them all at once with a Vector3

It's also a good habit to declare variables

local bodyVelocity = script.Parent.Parent.M1.BodyVelocity
bodyVelocity.velocity = Vector3.new(0, 0, 2)
wait(3)
bodyVelocity.velocity = Vector3.new(0, 0, -2)

To repeat it you would use a 'while' loop

while true do
    print("hello :c")
    wait()
end

That will repeat forever printing hello :c

We can put your code in a while loop and it'll repeat forever!

local bodyVelocity = script.Parent.Parent.M1.BodyVelocity
while true do
    bodyVelocity.velocity = Vector3.new(0, 0, 2)
    wait(3)
    bodyVelocity.velocity = Vector3.new(0, 0, -2)
    wait(3) --We need another wait here or else it's going to set it to '2' right away
end

In the comments you were asking how to repeat it so if you want to repeat it over and over again do this

local bodyVelocity = script.Parent.Parent.M1.BodyVelocity -- This is making the variable
while true do-- This is a loop that makes it repeat over and over
bodyVelocity.velocity = Vector3.new(0, 0, 2)
wait(3)-- This is waiting for it to repeat each time
bodyVelocity.velocity = Vector3.new(0, 0, -2)
end

Cant Test it right now but im pretty sure it works so hope this helpes:)