How do you assign a value to a random position in an array?

I am trying to make a Minesweeper game and I want to randomly place bombs across and array of buttons

So far my code for the array of buttons looks like this:

I want to have an array of buttons and just change the text of 10 of them, selected at random to display a B or a background image of a bomb.

int horizontal = 270;
        int vertical = 150;
        Button[] buttonArray = new Button[81];
        for (int i = 0; i < buttonArray.Length; i++)
        {
            buttonArray[i] = new Button();
            buttonArray[i].Size = new Size(20, 20);
            buttonArray[i].Location = new Point(horizontal, vertical);
            if ((i == 8) || (i == 17) || (i == 26) || (i == 35) ||
             (i == 53) || (i == 62) || (i == 71))
            {
                vertical = 150;
                horizontal = horizontal + 20;
            }
            else
                vertical = vertical + 20;
            this.Controls.Add(buttonArray[i]);
        }