for any object, when you're in the layout editor, you can right click an object and go to "Order"
to adjust the order of objects
under the sprite object, it's the options at the bottom of the actions
there is also a "get z index" expression for sprites
for this object in particular
you can check the option in the properties panel
and you can also change it with the Set Z-Order to object action
like I said before though, if you'd prefer to change the zorder according to other things to make letters weave in and out of pillars or something while they scroll, you can do a For Each Letter condition, and do whatever you want to the sprites