How do I randomly replace some element from a grid?

Ok, I thought about it and realized that I have no idea how to do so. This is my previous question on how to build a grid (already solved but needed to solve this): How to make a 20 rows x 50 columns grid in java

Now I need to replace 75 of these dots with landmines (%) randomly in the grid, since the game is gonna be brand new every time. But I have no idea how to do this.