Use user input to change board

I have a function that makes a board

 function userboard=userboard(n)
userboard = cell(n+1,n+1);
 % initialize the board
 for u=1:n+1
     for v=1:n+1
         if u==1
             if v<=n
                 userboard{u,v+1} = v;
                 userboard{v+1,u} = v;
             end
         elseif v==1
             % do nothing
         else
             userboard{u,v} = 'x';
         end
     end
 end

and I want to take the user inputs for x and y

x = input( 'Enter your x coordinate ' );
y = input( 'Enter your y coordinate ' );

and make those spots on the board a 0.

How do I do this?