im trying to figure this out pls help me

when [1 v] key pressed
switch costume to [insert costume here v]

For the future, these questions go into Help with Scripts.

If you mean switching to another coustume do what @Nes1Owe_Alt said but to switch the sprite do this:

Make Two Sprites, Sprite A and Sprite B (or whatever you want the names to be) and code this in Sprite A(the one you want to switch off of when the 1 key is pressed):

when green flag clicked
show
when [1] key pressed
hide

And For Sprite B(The one we are switching to):

when green flag clicked
hide
when [1] key pressed
show