Replace values in matrix with other values

I have a matrix with integers and I need to replace all appearances of 2 with -5. What is the most efficient way to do it? I made it the way below, but I am sure there is more elegant way.

a=[1,2,3;1,3,5;2,2,2]
ind_plain = find(a == 2)
[row_indx col_indx] = ind2sub(size(a), ind_plain)
for el_id=1:length(row_indx)
    a(row_indx(el_id),col_indx(el_id)) = -5;
end

Instead of loop I I seek for something like: a(row_indx,col_indx) = -5, which does not work.