Tilemap.SwapTile

public void SwapTile (Tilemaps.TileBase changeTile, Tilemaps.TileBase newTile);

Parameters

changeTile	Tile to swap.
newTile	Tile to swap to.

Description

Swaps all existing tiles of changeTile to newTile and refreshes all the swapped tiles.

// Change all occurences of tileA into tileB
using UnityEngine;
using UnityEngine.Tilemaps;

public class ExampleClass : MonoBehaviour
{
    public TileBase tileA;
    public TileBase tileB;

    void Start()
    {
        Tilemap tilemap = GetComponent<Tilemap>();
        tilemap.SwapTile(tileA, tileB);
    }
}