how to reduce the dimension of a feature space?

i have 260 sample,each sample has 320 feature (x is a matrix with 260 rows & 320 column).in order to improve my classification,i need to reduce these 320 column(i mean number of features).but i dont know how to do.when i use for example:

[pc,score,latent,tsquare] = princomp(X); red_dim = score(:,1:50);

how to reconstruct the matrix with fewer column? when i use : residuals = pcares(X,ndim) ,the dimension of residuals is the same of x !!!!

[Walter Roberson]

duplicates by later http://www.mathworks.co.uk/matlabcentral/answers/78042-how-to-reduce-the-dimension-of-a-feature-space