How do I replace player animations?

I want to replace the walking animation and the death animation.

When a character spawn, they come with an Animate script that creates and loads the preset animations. All you have to do it destroy that script when a character spawns and replace it with your edited version.

And for death, you replace shoulder and hip Motor6Ds with Glue welds for a rag doll.

Otherwise, remake all the joints when a character dies and run your death animation when the humanoid dies.