how do you make backdrops change like if you were scrolling

As far as I know, you can't. People do it by making big Sprites and moving them.

If it is something that is repetitive you could make a lot of backgrounds and animate them in a loop, This only lets you move in 1 dimension.

This should go in Help with Scripts.

But beforehand, to change a backdrop, you'd use either “next backdrop” or "switch to backdrop [ v]“. If you wanted to change the backdrop to give the appearance of scrolling, make a variable called ”Scroll X“, which should give you horizontal scrolling. Next, if a player were to move across the ground which scrolls, use the formula

((Scroll X) mod (A)) + B

where A is how many backdrops you want to cycle through and B is the starting backdrop number. Place this formula in the ”[ v]“ part of the ”switch backdrop to“ block. Now you'll want a basic script for your ”Scroll X" to change, so how about

set [Scroll X v] to [0]
forever
if <key [right arrow v] pressed?>
change [Scroll X v] by (1)
end if
if <key [left arrow v] pressed?>
change [Scroll X v] by (-1)
end if
end forever

I hope this helps!