how to select/change which sprite set is used from a multiple set spritesheet?

This is what works for me in this scenario. This way you can get all the Sprites inside the single Texture., First, you will need to get a "Resource" folder inside the "Asset" folder (Note: Anything within the Resource folder can be gathered within the script). Here is an example using

     public class CursorScript : MonoBehaviour
     {
         private SpriteRenderer spriteRender;
     
         private Sprite[] renderToSprite;
     
         // Start is called before the first frame update
         void Start()
         {
             //Get the "SpriteRender Component to modified the image of the cursor.
             spriteRender = gameObject.GetComponent<SpriteRenderer>();
     
             //Set the name of the Image Sprite.
             string multiSpriteName = spriteRender.sprite.texture.name;
     
             //Set the location A.K.A the path of where the multi-Sprite Image is located 
 (Note: The image has to be inside the "Resource" Folder).
             string path = "Sprites/UI/";
     
             //Get all Sprites that fit the requirements to cast into the "RenderToSprite".
             renderToSprite = Resources.LoadAll<Sprite>(path + multiSpriteName);
     
             //Render a new sprite selected in the "renderToSprite[Array]".
             spriteRender.sprite = renderToSprite[1];
     
         }
     }

This is what works for me in this scenario. This way you can get all the Sprites inside the single Texture.