create logic for LevelUp() method in unity, so each 10 LinesCleared the fallTime decreses by 0.2f; and the max it can decrease to is 0.1f; :

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using TMPro;
using UnityEngine.UI;

public class TetrisBlock : MonoBehaviour
{
    public Vector3 rotationPoint;
    private float previousTime;
    public float fallTime = 0.8f;
    public static int height = 23;
    public static int width = 10;
    private static Transform[,] grid = new Transform[width, height];
    public TMP_Text Lines_Cleared;
    public static int LinesCleared;

    public int linesPerLevel = 10;
    public float fallTimeDecrease = 0.2f;
    public float minFallTime = 0.1f;
    private int level = 0;


    void Start()
    {


    }

    void Update()
    {
        LevelUp();
        if(Input.GetKeyDown(KeyCode.LeftArrow) || Input.GetKeyDown(KeyCode.A))
        {
            transform.position += new Vector3(-1, 0, 0);
            if(!ValidMove())
                transform.position -= new Vector3(-1, 0, 0);
        }
        else if (Input.GetKeyDown(KeyCode.RightArrow) || Input.GetKeyDown(KeyCode.D))
        {
            transform.position += new Vector3(1, 0, 0);
            if (!ValidMove())
                transform.position -= new Vector3(1, 0, 0);
        }
        else if (Input.GetKeyDown(KeyCode.X))
        {
            transform.RotateAround(transform.TransformPoint(rotationPoint), new Vector3(0,0,1), 90);
            if (!ValidMove())
                transform.RotateAround(transform.TransformPoint(rotationPoint), new Vector3(0, 0, 1), -90);
        }


        if (Time.time - previousTime > (Input.GetKey(KeyCode.DownArrow) || Input.GetKeyDown(KeyCode.S) ? fallTime / 10 : fallTime))
        {
            transform.position += new Vector3(0, -1, 0);
            if (!ValidMove())
            {
                transform.position -= new Vector3(0, -1, 0);
                AddToGrid();
                CheckForLines();

                this.enabled = false;
                FindObjectOfType<SpawnTetromino>().NewTetromino();

            }
            previousTime = Time.time;
        }
    }

    void CheckForLines()
    {
        for (int i = height-1; i >= 0; i--)
        {
            if(HasLine(i))
            {
                LinesCleared++;
                DeleteLine(i);
                RowDown(i);
                UpdateUi();
            }
        }
    }

    bool HasLine(int i)
    {
        for(int j = 0; j< width; j++)
        {
            if (grid[j, i] == null)
                return false;
        }

        return true;
    }

    void DeleteLine(int i)
    {
        for (int j = 0; j < width; j++)
        {
            Destroy(grid[j, i].gameObject);
            grid[j, i] = null;
        }
    }

    void RowDown(int i)
    {
        for (int y = i; y < height; y++)
        {
            for (int j = 0; j < width; j++)
            {
                if(grid[j,y] != null)
                {
                    grid[j, y - 1] = grid[j, y];
                    grid[j, y] = null;
                    grid[j, y - 1].transform.position -= new Vector3(0, 1, 0);
                }
            }
        }
    }




    void AddToGrid()
    {
        foreach (Transform children in transform)
        {
            int roundedX = Mathf.RoundToInt(children.transform.position.x);
            int roundedY = Mathf.RoundToInt(children.transform.position.y);

            grid[roundedX, roundedY] = children;
        }
    }

    bool ValidMove()
    {
        foreach (Transform children in transform)
        {
            int roundedX = Mathf.RoundToInt(children.transform.position.x);
            int roundedY = Mathf.RoundToInt(children.transform.position.y);

            if(roundedX < 0 || roundedX >= width || roundedY < 0 ||roundedY >= height)
            {
                return false;
            }

            if (grid[roundedX, roundedY] != null)
                return false;
        }

        return true;
    }

    public static void UpdateUi()
    {
        TMP_Text linesClearedText = GameObject.Find("Lines_Cleared").GetComponent<TextMeshProUGUI>();
        linesClearedText.text = "Lines Cleared: " + LinesCleared.ToString();
        Debug.Log(linesClearedText.text);
    }

    void LevelUp()
    {
        
    }
}