Storing a variable and calling it out again to use and plot

From what i understand in this question, I need to store the current temperature,T of the following into a new variable T_old and update each value of T accordingly. But when i plotted it out there is an aerror. Also i cannot seem to store the T into T-old and calling it out to use. This is my codes for the first part.

x =-0.8:0.8:5;
dt=1000;
T=ones(1,5); T(1,5)=40;
figure
plot(x,T)
title('Temperature distribution between x=0 and x=0.8 ')
xlabel('x')
ylabel('Temperature')
hold on;
T=T_old