The most straightforward approach is probably to build a vector of month IDs, and then to loop over months and update values. Something along these lines (assuming that your 35040x2 array is named data):
[~, monthId] = datevec( data(:,1) ) ;
for m = 1 : 12
data(monthId==m,2) = data(monthId==m,2) + vector(m) ;
end
You can make it more concise and eliminate the loop as follows (but it is a bit less easy to understand):
[~, monthId] = datevec( data(:,1) ) ;
data(:,2) = data(:,2) + vector(monthId) ;
