How to change the background as the first person character goes up?

I am making an interactive space game where a first person character goes up to space from the ground. As the character goes up, i want to change the texture from sky blue to a space/ nebula texture.
How is it possible?
How do i change the background from a blue sky to space?

The “blue sky” in your project is called SKYBOX. Read more in this link: Unity Manual for Skyboxes

You can make a simple script using the OnTriggerEnter function. Read more in this link: Unity Docs about OnTrigger

Then you can make a simple script for when the player get into a object’s trigger that will be on X height, the Skybox’s materials will be changed. Learn how to change materials here: Unity Docs about material changing
Alternatively, you can just literally search for it: