Did you try using minus instead of plus?
yNew = y + offset; % What you've already done.
% Now subtract from the signal using the "minus" operation
yNew2 = yNew - offset; % Should equal the original y signal.
offset can be whatever you want. If you want it to be the average of the signal do
offset = mean(y);
offset = mean(yNew); % Or this if you want.
