Your figure does not show "two graphs having upper portion same but different lower portion as in the figure". In fact they're different everywhere. The lower portion is actually more similar than the upper portion. Anyway to shift the brown signal up you do
brownMax = max(yBrown);
blueMax = max(yBlue);
% Now shift and put the shifted copy in a new variable.
yBrown2 = yBrown - brownMax + blueMax;
Then plot yBrown2.
