The simplest (but not the most efficient) way to do this would be to inititalize a for loop and run through all the elements one-by-one.
for ii = 1:length(y)
if y(ii) < 0
y(ii) = randi([0, 2]);
The function randi() generates a random number bertween the limits given in the square brackets, in this case 0 and 2.
A better (faster and more efficient in terms of lines of code) way to do this would be
y (y < 0) = randi([0, 2]);