I have a shader and I want it to get more extreme with the players speed. How would I edit this shader's parameters in runtime?
You need to get a reference to the material. If it's a CanvasItem (2D) you can get it with material. If it's a MeshInstance, you can use its GeometryInstance.material_override or Mesh.surface_get_material or PrimitiveMesh.material. I've linked to the class docs if you need details.
After that it depends on what kind of material it is:
ShaderMaterial: You can change any uniform parameter with set_shader_param()
SpatialMaterial: set the parameter name directly: