Hi you need to set up a grid for each point that you want to evaluate. Afterwards you need to evaluate your function at each point and store it in a matrix. See below for a demonstration.
% define ranges for x and t
x = linspace(0,pi,60);
t = [linspace(0,0.05,20), linspace(0.5,5,10)];
% obtain the grid for the surface plot
[X,T] = meshgrid(x,t);
% initialize an array for the solution
U = zeros(size(X));
% set up the range for n
n = 1 : 1000;
for i = 1:numel(X)
% extract the current value of t and x (this is not really needed, just
% for readability)
xi = X(i);
ti = T(i);
U(i) = sum( 2/pi.*((2*sin(pi.*n/2)-sin(n.*pi))./n.^2).*(exp((-n.^2).*ti).*sin(n.*xi)) );
end
% create the figure
figure
surf(X,T,U)
grid on
xlabel('x'); ylabel('t'); zlabel('u')
view(3)
