Assumption : there is on only one decreasing region in your data points.
x = -2:0.1:6;
y = x.^3-5*x.^2+5*x-2;
D = diff(y);
decreaseStart = find(D<0,1,'first');
decreaseEnd = find(D<0,1,'last');
plot(x,y)
hold on
plot(x(decreaseStart:decreaseEnd),y(decreaseStart:decreaseEnd),'r');
